Electrolyte additives for electrochemical devices

ABSTRACT

A system and method for stabilizing electrodes against dissolution and/or hydrolysis including use of cosolvents in liquid electrolyte batteries for three purposes: the extension of the calendar and cycle life time of electrodes that are partially soluble in liquid electrolytes, the purpose of limiting the rate of electrolysis of water into hydrogen and oxygen as a side reaction during battery operation, and for the purpose of cost reduction.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 15/442,634 filed 25 Feb. 2017; this application is acontinuation-in-part of U.S. patent application Ser. No. 13/892,982filed 13 May 2013 which claims benefit of U.S. Patent Application No.61/722,049 filed 2 Nov. 2012; and this application is acontinuation-in-part of U.S. patent application Ser. No. 15/062,171filed 6 Mar. 2016 which is a continuation of U.S. patent applicationSer. No. 14/231,571 (now U.S. Pat. No. 9,287,589) filed 31 Mar. 2014which claims benefit of U.S. Patent Application No. 61/810,684 filed 10Apr. 2013, the contents of which are all hereby expressly incorporatedby reference thereto in their entireties for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under ARPA-E Award No.DE-AR000300 With Alveo Energy, Inc., awarded by DOE. The government hascertain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to electrochemical devices, andmore specifically, but not exclusively, to balancing electrode potentialof electrodes in an electrochemical device in which at least oneelectrode includes a material in which potential may vary by state ofcharge.

The present invention also relates generally to rechargeable energyaccumulators, and more specifically, but not exclusively, tostabilization of electrodes used with aqueous electrolytes and even moreparticularly to stabilization of electrodes used with aqueouselectrolytes as part of an electrochemical cell.

BACKGROUND OF THE INVENTION

The subject matter discussed in the background section should not beassumed to be prior art merely as a result of its mention in thebackground section. Similarly, a problem mentioned in the backgroundsection or associated with the subject matter of the background sectionshould not be assumed to have been previously recognized in the priorart. The subject matter in the background section merely representsdifferent approaches, which in and of themselves may also be inventions.

A wide variety of battery technologies have been developed for portableand stationary applications, including lead acid, lithium-ion,nickel/metal hydride, sodium sulfur, and flow batteries, among others.Not one of these technologies is commonly used for applications relatedto the stabilization and reliability of the electric grid due toexorbitantly high cost, poor cycle and calendar lifetime, and low energyefficiency during rapid cycling. However, the development of lower cost,longer lived batteries is likely needed for the grid to remain reliablein spite of the ever-increasing deployment of extremely volatile solarand wind power.

Existing battery electrode materials cannot survive for enough deepdischarge cycles for the batteries containing them to be worth theirprice for most applications related to the electric grid. Similarly, thebatteries found in electric and hybrid electric vehicles are long livedonly in the case of careful partial discharge cycling that results inheavy, large, expensive battery systems. The performance of mostexisting battery electrode materials during fast cycling is limited bypoor kinetics for ion transport or by complicated, multi-phaseoperational mechanisms.

The use of Prussian Blue analogues (PBAs), which are a subset of a moregeneral class of transition metal cyanide coordination compounds(TMCCCs) of the general chemical formula A_(x)P_(y)[R(CN)₆]_(z).nH₂O(A=alkali cation, P and R=transition metal cations, 0≤x≤2, 0≤y≤4, 0≤z≤1,0≤n), has been previously demonstrated as electrodes in aqueouselectrolyte batteries. TMCCC electrodes have longer deep discharge cyclelife and higher rate capability than other intercalation mechanismelectrodes, and they enjoy their highest performance in aqueouselectrolytes. TMCCC cathodes rely on the electrochemical activity ofiron in Fe(CN)₆ complexes at high potentials. TMCCC anodes, on the otherhand, contain electrochemically active, carbon-coordinated manganese orchromium.

The development of a symmetric battery in which both the anode and thecathode are each a TMCCC is desirable because TMCCCs have longer cyclelife and can operate at higher charge/discharge rates than otherelectrode systems. If one TMCCC electrode were to be paired with adifferent kind of electrode, it is likely that the full battery wouldnot last as long, or provide the same high-rate abilities as a symmetriccell containing a TMCCC anode and a TMCCC cathode.

TMCCC cathodes are well understood, and the operation of a TMCCC cathodefor over 40,000 deep discharge cycles has been previously demonstrated.These cathodes typically operate at about 0.9 to 1.1 V vs. the standardhydrogen electrode (SHE). One challenge for the development of practicalbatteries using TMCCC cathodes is their trace solubility in aqueouselectrolytes. Their partial dissolution into the battery electrolyte canresult in a decrease in battery charge capacity due to mass loss fromthe electrodes and a decrease in efficiency due to side reactions withthe cathode's dissolution products.

In some embodiments, an order of production and assembly of componentsof an electrochemical device may affect performance metrics of thecompleted electrochemical device. For example, in some instances of acosolvent electrochemical device, it may be better to add a chemicalspecies to an electrolyte of the electrochemical device before addingthe electrolyte to the rest of the electrochemical device.

The development of a TMCCC anode has proven much more challenging thanthat of TMCCC cathodes because these materials typically have reactionpotentials either near 0 V or below −0.5 V vs. SHE, but not in the rangebetween −0.5 V and 0 V that is most desirable in aqueous electrolytes,and because they operate only in a narrow pH range without rapidhydrolysis to manganese dioxide phases. As the useful electrochemicalstability window of aqueous electrolytes at approximately neutral pH(pH=5-8) extends from about −0.4 V to 1 V vs. SHE, an anode reactionpotential of 0 V results in a cell voltage lower than the maximum thatis possible without decomposition of water. But, in the case of an anodereaction potential below −0.5 V vs. SHE, the charge efficiency of theanode can be poor due to rapid hydrolysis of water to hydrogen gas.Finally, if the Mn(CN)₆ groups in the TMCCC anode hydrolyze, thecapacity of the electrode is rapidly lost.

For purposes of this application, electrode materials may be dividedinto two classes: 1) electrode potential is constant with respect tostate of charge; and 2) electrode potential varies with respect to stateof charge.

For an electrochemical cell using electrodes of the first class, thereis no concern about unbalanced potentials on the electrode are balancedacross a range of charge of the cell.

However, for an electrochemical cell using one or more electrodes of thesecond class, there is a possibility that there could be unbalancedpotentials on the electrode, particularly in an event that the cell isnot at maximum charge. Unbalanced potentials reduce an energy density ofthe cell.

Commonly used materials for electrodes, such as lithium and graphite,are materials of the first class. There are materials of the secondclass that offer some improvements over these more conventionalmaterials. However electrochemical cells made with the materials of thesecond class may have a degraded performance in other areas, includingthe possibility of the unbalanced potentials.

There could be advantages to addressing the possible degradation whenusing electrode materials of the second class, such as improving energydensity while gaining the desired advantages of the alternativeelectrode materials or for slowing and/or preventing dissolution ofelectrodes into an operating electrolyte to extend a calendar life ofthe electrodes.

BRIEF SUMMARY OF THE INVENTION

Disclosed is a system and method for addressing the possible degradationwhen using electrode materials of the second class, such as improvingenergy density while gaining the desired advantages of the alternativeelectrode materials and/or slowing and/or preventing dissolution ofelectrodes into an operating electrolyte to extend a calendar life ofthe electrodes. The following summary of the invention is provided tofacilitate an understanding of some of technical features related to useof cosolvent electrolytes for more efficient and durable batteries, andis not intended to be a full description of the present invention and/orstabilization of TMCCC/PBA battery electrodes. A full appreciation ofthe various aspects of the invention can be gained by taking the entirespecification, claims, drawings, and abstract as a whole. The presentinvention is applicable to other electrode types in addition to TMCCCcathodes and/or anodes, to other electrochemical devices in addition tofull, partial, and/or hybrid battery systems including a liquidelectrolyte, and to other cell chemistries, materials, and analogues.

Some examples in this patent application concern the use of solvents andcosolvents in liquid electrolyte batteries for multiple purposes: theextension of the calendar and cycle life time of electrodes that arepartially soluble in liquid electrolytes, the purpose of limiting therate of electrolysis of water into hydrogen and oxygen as a sidereaction during battery operation, and for the purpose of costreduction. Cosolvents are when two liquids are combined into a singlesolution, as in the case of water and ethanol in wine, which may alsocontain dissolved compounds such as salts. Herein is demonstrated autility of these cosolvent electrolytes using the model system of anaqueous sodium ion electrolyte battery containing TMCCC electrodes, butthe benefits of cosolvents to the performance of liquid electrolytebatteries apply generally to other electrode and battery systems aswell. One cost benefit occurs because an organic cosolvent as disclosedherein allows one to have a higher voltage before water is quickly splitinto hydrogen and oxygen. When the organic cosolvent is relativelyinexpensive, and the electrodes are the same materials (as in someembodiments disclosed herein when the anode has two different reactionpotentials), then the organic cosolvent lets the electrochemical devicehave a higher voltage for about the same materials cost. Energy is equalto the product of the charge and the voltage, so a higher voltageelectrochemical cell that gets more energy from the same materials willtherefore have a lower cost/energy.

Embodiments of the present invention broadly includes a general conceptof the use of cosolvents in liquid electrolyte batteries, particularly,but not exclusively, in several areas, including: first, the concept ofusing cosolvents to protect TMCCC electrodes from dissolution and/orhydrolysis, and second, the ability to use a hexacyanomanganate-basedTMCCC anode with a reaction potential so low that it can only be usedwhen reduction of water to hydrogen gas is suppressed (as is the case,for example, when a cosolvent is used as herein described).

Included herein is description of a novel method for the stabilizationof TMCCC electrodes against dissolution and hydrolysis, whilesimultaneously suppressing hydrogen generation at the anode: for examplean addition of a cosolvent to an aqueous electrolyte. A cosolventelectrolyte is one in which multiple liquid solvents are combined toform a single liquid phase, in which the electrolyte salt and anyadditional additives are then dissolved. The presence of a cosolvent candrastically change the solubility and stability of materials includingboth TMCCCs and electrolyte salts. The proper choice of cosolvent slowsor prevents the dissolution and/or hydrolysis of TMCCC electrodes, andit allows for the high-efficiency operation of TMCCC anodes withreaction potentials below −0.5 V vs. SHE. The final result is anelectrochemical device that operates at voltages of nearly double thosethat can be achieved in simple aqueous electrolytes, with longerelectrode cycle and calendar lives.

Some embodiments of the present invention may include an electrochemicaldevice including at least a pair of electrodes in chemical communicationwith one or more electrolytes, one, some, or all of the electrodes mayeach include a variable potential material, each such electrodeincluding the same or different variable potential material, and one ormore additives to the electrochemical device that each participates in alimited side-reaction with one or more electrodes having variablepotential material. In response to charging the electrochemical device,each limited side-reaction degrades charging of the related electrode(s)for a limited duration. Those electrodes that do not participate in oneof the limited side-reactions may begin charging immediately at fullcoulombic efficiency. Each electrode that is participating in a limitedside-reaction charges more slowly due to degraded coulombic efficiency,for the duration of each applicable limited side-reaction. As eachlimited side-reaction completes, the associated electrode may then begincharging at full coulombic efficiency. Proper configuration andcoordination of appropriate limited side-reactions allow differentelectrodes to be adjustably charged to different potentials from thesame charging source.

A battery (cell) that comprises an electrolyte and two electrodes (ananode and a cathode), one or both of which is a TMCCC material of thegeneral chemical formula A_(x)P_(y)[R(CN)_(6-j)L_(j)]_(z).nH₂O, where: Ais a monovalent cation such as Na⁺, K⁺, Li⁺, or NH₄ ⁺, or a divalentcation such as Mg²⁺ or Ca²⁺; P is a transition metal cation such asTi³⁺, Ti⁴⁺, V²⁺, V³⁺, Cr²⁺, Cr³⁺, Mn⁺, Mn²⁺, Mn³⁺, Fe²⁺, Fe³⁺, Co²⁺,Co³⁺, Ni²⁺, Cu⁺, Cu²⁺, or Zn²⁺, or another metal cation such as Al³⁺,Sn²⁺, In³⁺, or Pb²⁺; R is a transition metal cation such as V²⁺, V³⁺,Cr²⁺, Cr³⁺, Mn²⁺, Mn³⁺, Fe²⁺, Fe³⁺, CO²⁺, Co³⁺, Ru²⁺, Ru³⁺, Os²⁺, Os³⁺,Ir²⁺, Ir³⁺, Pt²⁺, or Pt³⁺; L is a ligand that may be substituted in theplace of a CN⁻ ligand, including CO (carbonyl), NO (nitrosyl), or Cl⁻;0≤x≤2; 0<y≤4; 0<z≤1; 0≤j≤6; and 0≤n≤5; and where the electrolytecontains water, one or more organic cosolvents, and one or more salts,where: the electrolyte is a single phase.

A rechargeable electrochemical cell, includes a positive electrode; anegative electrode; and an electrolyte having a total electrolyte volumeV including a first quantity of water comprising a first fraction V1 ofthe total electrolyte volume V and including a second quantity of one ormore organic cosolvents together comprising a second fraction V2 of thetotal electrolyte volume V; wherein V1/V>0.02; wherein V2>V1; wherein aparticular one electrode of the electrodes includes a transition metalcyanide coordination compound (TMCCC) material; and wherein theelectrolyte is a single phase.

A rechargeable electrochemical cell, includes a positive electrode; anegative electrode; and an electrolyte having a total electrolyte weightW including a first quantity of water comprising a first fraction W1 ofthe total electrolyte weight W and including a second quantity of one ormore organic cosolvents together comprising a second fraction W2 of thetotal electrolyte weight W; wherein W1/W>0.02; wherein W2>W1; wherein aparticular one electrode of the electrodes includes a transition metalcyanide coordination compound (TMCCC) material; and wherein theelectrolyte is a single phase.

A method for operating a rechargeable electrochemical cell having anegative electrode disposed in a single phase liquid electrolyte of atotal electrolyte quantity Q including at least a total quantity Q1 ofwater wherein Q1/Q is approximately 0.02 or greater and wherein anelectrolysis of the total quantity Q1 of water below a first potentialV1 initiates a production of hydrogen gas at a first rate R1, includinga) exchanging ions between the negative electrode and the liquidelectrolyte at an electrode potential VE, VE<V1; and b) producinghydrogen gas at a second rate R2 less than R1 responsive to theelectrode potential VE; wherein an electrolysis of the total electrolytequantity Q a second quantity of one or more organic cosolvents togethercomprising a second fraction Q2 of the total electrolyte quantity Qbelow a second potential V2 initiates the production of hydrogen gas atthe first rate R1, V2<V1; and wherein VE>V2.

A rechargeable electrochemical device, includes a first electrode; asecond electrode; an electrolyte coupled with the electrodes; and afirst additive in communication with the electrolyte; wherein a firstparticular one electrode of the electrodes includes a first variablepotential material; and wherein the first additive participates in afirst predetermined side-reaction with a first single one of theelectrodes degrading a charging efficiency of the first single one ofthe electrodes for a duration of the first predetermined side-reaction.

A method for reducing a relative state-of-charge imbalance of a set ofelectrodes of a rechargeable electrochemical device during a rechargingprocess, the set of electrodes coupled to an electrolyte and wherein atleast one electrode of the set of electrodes includes a first variablepotential material, including a) performing the recharging process for arecharging duration which charges the electrodes at different relativerates to tend to produce a relative state-of-charge imbalance for theset of electrodes; and b) reducing the relative state-of-chargeimbalance by interfering with a charging of at least one electrode ofthe set of electrodes. In an embodiment, the reducing step b) mayinclude b1) communicating an additive to the electrolyte to induce apredetermined side-reaction with the at least one electrode includingthe first variable potential material; and b2) degrading a chargingefficiency of the at least one electrode for a duration of thepredetermined side-reaction.

A battery (cell) including: an electrolyte (which may be aqueous orquasi-aqueous) and two electrodes (an anode and a cathode), one or bothof which is a TMCCC material of the general chemical formulaA_(x)P_(y)[R(CN)_(6-j)L_(j)]_(z).nH₂O, where: A is a monovalent cationsuch as Na⁺, K⁺, Li⁺, or NH₄ ⁺, or a divalent cation such as Mg²⁺ orCa²⁺; P is a transition metal cation such as V²⁺, V³⁺, Cr²⁺, Cr³⁺, Mn⁺,Mn²⁺, Mn³⁺, Fe²⁺, Fe³⁺, Co²⁺, Co³⁺, Ni²⁺, Cu⁺, Cu²⁺, or Zn²⁺, or anothermetal cation such as Al³⁺, Sn²⁺, In³⁺, or Pb²⁺; R is a transition metalcation such as V²⁺, V³⁺, Cr²⁺, Cr³⁺, Mn⁺, Mn²⁺, Mn³⁺, Fe²⁺, Fe³⁺, Co²⁺,Co³⁺, Ru²⁺, Ru³⁺, Os²⁺, Os³⁺, Ir²⁺, Ir³⁺, Pt²⁺, or Pt³⁺; L is a ligandthat may be substituted in the place of a CN⁻ ligand, including CO(carbonyl), NO (nitrosyl), or Cl⁻; 0≤j≤6; 0≤x≤2; 0<y≤4; 0<z≤1; and0≤n≤5.

A battery including an electrolyte in contact with two electrodes, inwhich a conformal coating of a TMCCC of the general chemical formuladescribed herein on the surface of one or more of the electrodesprevents dissolution of that electrode into the electrolyte.

A battery including an electrolyte in contact with two electrodes, inwhich a conformal coating of a TMCCC of the general chemical formuladescribed herein on the surface of the individual particles of theelectrochemically active material within the electrode preventsdissolution of that material into the electrolyte.

A battery including an electrolyte in contact with two electrodes, inwhich a conformal coating of a mixed conducting polymer such aspolypyrrole on the surface of one or more of the electrodes preventsdissolution of that electrode into the electrolyte.

A battery including an electrolyte in contact with two electrodes, inwhich a conformal coating of a mixed conducting polymer such aspolypyrrole on the surface of the individual particles of theelectrochemically active material within the electrode preventsdissolution of that material into the electrolyte.

An electrochemical apparatus including an operating aqueous electrolyteincluding a quantity of water, a plurality of ions, and an electrolyteadditive distributed in the quantity of water; and a first electrodedisposed in the operating aqueous electrolyte, the first electrodeincluding a first TMCCC material having a general chemical formulaA_(x)P_(y)[R(CN)_(6-j)L_(j)]_(z).nH₂O, where: A is a cation, P is ametal cation, R is a transition metal cation, and L is a ligandsubstitutable in the place of a CN⁻ ligand, and 0≤j≤6, 0≤x≤2, 0<y≤4,0<z≤1, and 0≤n≤5, wherein the first TMCCC material has a first specificchemical formula conforming to the general chemical formula including afirst particular cation P₁ and a first particular cation R₁, wherein thefirst electrode has a first rate of electrochemical capacity loss whendisposed in the operating aqueous electrolyte, and wherein the firstTMCCC material has a second rate of electrochemical capacity loss whendisposed in a second aqueous electrolyte consisting of water and theplurality of ions without the electrolyte additive; wherein the firstrate of electrochemical capacity loss is less than the second rate ofelectrochemical capacity loss.

A method for manufacturing an electrochemical apparatus including afirst electrode having a first TMCCC material with a general chemicalformula A_(x)P_(y)[R(CN)_(6-j)L_(j)]_(z).nH₂O, where: A is a cation, Pis a metal cation, R is a transition metal cation, and L is a ligandsubstitutable in the place of a CN⁻ ligand, and 0≤j≤6, 0≤x≤2, 0<y≤4,0<z≤1, and 0≤n≤5, wherein the first TMCCC material has a first specificchemical formula conforming to the general chemical formula including afirst particular cation P₁ and a first particular cation R₁, and whereinthe first TMCCC material has a rate of electrochemical capacity losswhen disposed in an aqueous electrolyte including a plurality of ions,the method including (a) disposing the first electrode in the aqueouselectrolyte; and (b) decreasing the rate of electrochemical capacityloss by distributing an electrolyte additive into the aqueouselectrolyte.

Any of the embodiments described herein may be used alone or togetherwith one another in any combination. Inventions encompassed within thisspecification may also include embodiments that are only partiallymentioned or alluded to or are not mentioned or alluded to at all inthis brief summary or in the abstract. Although various embodiments ofthe invention may have been motivated by various deficiencies with theprior art, which may be discussed or alluded to in one or more places inthe specification, the embodiments of the invention do not necessarilyaddress any of these deficiencies. In other words, different embodimentsof the invention may address different deficiencies that may bediscussed in the specification. Some embodiments may only partiallyaddress some deficiencies or just one deficiency that may be discussedin the specification, and some embodiments may not address any of thesedeficiencies.

Other features, benefits, and advantages of the present invention willbe apparent upon a review of the present disclosure, including thespecification, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, in which like reference numerals refer toidentical or functionally-similar elements throughout the separate viewsand which are incorporated in and form a part of the specification,further illustrate the present invention and, together with the detaileddescription of the invention, serve to explain the principles of thepresent invention.

FIG. 1 illustrates a schematic of batteries using the higher and loweranode reactions for CuHCF and MnHCMn;

FIG. 2 illustrates a unit cell of the TMCCC crystal structure;

FIG. 3 illustrates a cyclic voltammogram of MnHCMn in cosolvents;

FIG. 4 illustrates a cyclic voltammogram of MnHCMn in cosolvents;

FIG. 5 illustrates a cyclic voltammogram of MnHCMn in cosolvents;

FIG. 6 illustrates a cyclic voltammogram of MnHCMn in cosolvents;

FIG. 7 illustrates a cyclic voltammogram and integrated current ofMnHCMn in 90% MeCN;

FIG. 8 illustrates a cyclic voltammogram of CuHCF in cosolvents;

FIG. 9 illustrates a cyclic voltammogram of MnHCMn in 90% or 100% MeCN;

FIG. 10 illustrates a cycle life of MnHCMn in half cells;

FIG. 11 illustrates a set of potential profiles of MnHCMn in half cells;

FIG. 12 illustrates a cycle life of CuHCF in half cells;

FIG. 13 illustrates a set of GCPL vs. time profiles of MnHCMn vs. CuHCFin the full cell;

FIG. 14 illustrates a full cell voltage profile;

FIG. 15 illustrates a full cell voltage profile of the cell illustratedin FIG. 13;

FIG. 16 illustrates a representative secondary electrochemical cellschematic having one or more TMCCC electrodes disposed in contact with acosolvent electrolyte as described herein; and

FIG. 17-FIG. 30 illustrate seven pairs of charts corresponding toExample A3-Example A9, each pair of charts including an electrodespotential chart and a full cell voltage chart;

FIG. 17-FIG. 18 illustrate a first pair of charts for Example A3comparing a control (no additive) to a Cu(NO₃)₂ additive;

FIG. 17 illustrates an electrode potentials chart for Example A3; and

FIG. 18 illustrates a cell voltage chart for Example A3; and

FIG. 19-FIG. 20 illustrate a second pair of charts for Example A4comparing a control (no additive) to a Benzoquinone additive;

FIG. 19 illustrates an electrode potentials chart for Example A4; and

FIG. 20 illustrates a cell voltage chart for Example A4; and

FIG. 21-FIG. 22 illustrate a third pair of charts for Example A5comparing a control (no additive) to a Hydroquinone additive;

FIG. 21 illustrates an electrode potentials chart for Example A5; and

FIG. 22 illustrates a cell voltage chart for Example A5; and

FIG. 23-FIG. 24 illustrate a fourth pair of charts for Example A6comparing a control (no additive) to a Ferrocene additive;

FIG. 23 illustrates an electrode potentials chart for Example A6; and

FIG. 24 illustrates a cell voltage chart for Example A6; and

FIG. 25-FIG. 26 illustrate a fifth pair of charts for Example A7comparing a control (no additive) to a Cu(NO₃)₂ additive;

FIG. 25 illustrates an electrode potentials chart for Example A7; and

FIG. 26 illustrates a cell voltage chart for Example A7; and

FIG. 27-FIG. 28 illustrate a sixth pair of charts for Example A8comparing a control (no additive) to an Oxalic acid additive;

FIG. 27 illustrates an electrode potentials chart for Example A8; and

FIG. 28 illustrates a cell voltage chart for Example A8; and

FIG. 29-FIG. 30 illustrate a seventh pair of charts for Example A9comparing a control (no additive) to a Pyrrole additive;

FIG. 29 illustrates an electrode potentials chart for Example A9;

FIG. 30 illustrates a cell voltage chart for Example A9; and

FIG. 31 illustrates a set of charts for charging and discharging under aset of different cases;

FIG. 32 illustrates a unit cell of the Prussian Blue crystal structure;

FIG. 33 illustrates an X-ray diffraction spectrum of CuHCF;

FIG. 34 illustrates a micrograph of CuHCF;

FIG. 35 illustrates X-ray diffraction spectra of MnHCMn;

FIG. 36 illustrates a micrograph of MnHCMn;

FIG. 37 illustrates baseline/control electrochemical cycling of CuHCF;

FIG. 38 illustrates a UV-visible spectrum of CuHCF in water and 1 M KNO₃pH=2;

FIG. 39 illustrates an ultraviolet-visible absorbance spectrum of CuHCFin water and 10 mM Cu²⁺;

FIG. 40 illustrates the cycle life of CuHCF in 1 M KNO₃ pH=2 with andwithout CU²⁺ added;

FIG. 41 illustrates galvanostatic cycling of CuHCF/Cu²⁺/Cumetal in 2sub-figures, including FIG. 41a and FIG. 41 b;

FIG. 41a illustrates potential profiles of the copper hexacyanoferratecathode and the copper anode, and the full cell voltage, duringgalvanostatic cycling at a 1C rate in 1 M KNO₃; and

FIG. 41b illustrates the same data, plotted as a function of thespecific capacity of the copper hexacyanoferrate cathode;

FIG. 42 illustrates cyclic voltammetry of CuHCF and PB/BG;

FIG. 43 illustrates capacity retention of PB/CuHCF and CuHCF;

FIG. 44 illustrates capacity retention of CuHCF w/K⁺ in PB dep solution;

FIG. 45 illustrates potential profiles of CuHCF and Prussian Blue-coatedCuHCF electrodes;

FIG. 46 illustrates morphologies of bare and Prussian Blue-coated CuHCFelectrodes in two sub-figures, including FIG. 46a and FIG. 46 b;

FIG. 46a illustrates scanning electron microscopy of a freshly depositedslurry electrode of copper hexacyanoferrate (80%), carbon black (10%),and polyvinylidene difluoride (10%) on a carbon cloth substrate; and;

FIG. 46b illustrates the same sample, after electrochemical reduction,followed by 40 minutes of exposure to a 2 mM aqueous solution of Fe(CN)₃and K₃Fe(CN)₆;

FIG. 47 illustrates cycle life of CuHCF with PB coating on theparticles;

FIG. 48 illustrates potential profiles of CuHCF with PB coating on theparticles in two sub-figures, including FIG. 48a and FIG. 48 b;

FIG. 48a illustrates the potential profiles of electrodes containinguntreated copper hexacyanoferrate, and copper hexacyanoferratenanoparticles coated with Prussian Blue, during galvanostatic cycling ata 1C rate in 1 M KNO₃ (pH=2); and

FIG. 48b illustrates Galvanostatic cycling of an electrode containingPrussian-Blue coated copper hexacyanoferrate nanoparticles over a widerpotential range;

FIG. 49 illustrates cycle life of CuHCF with PPy coating on theparticles; and

FIG. 50 illustrates potential profiles of CuHCF with PPy coating on theparticles.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention provide a system and method foraddressing the possible degradation when using electrode materials ofthe second class, such as improving energy density while gaining thedesired advantages of the alternative electrode materials. The followingdescription is presented to enable one of ordinary skill in the art tomake and use the invention and is provided in the context of a patentapplication and its requirements.

Various modifications to the preferred embodiment and the genericprinciples and features described herein will be readily apparent tothose skilled in the art. Thus, the present invention is not intended tobe limited to the embodiment shown but is to be accorded the widestscope consistent with the principles and features described herein.

Definitions

The following definitions apply to some of the aspects described withrespect to some embodiments of the invention. These definitions maylikewise be expanded upon herein.

As used herein, the singular terms “a,” “an,” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to an object can include multiple objects unless thecontext clearly dictates otherwise.

Also, as used in the description herein and throughout the claims thatfollow, the meaning of “in” includes “in” and “on” unless the contextclearly dictates otherwise.

As used herein, the term “set” refers to a collection of one or moreobjects. Thus, for example, a set of objects can include a single objector multiple objects. Objects of a set also can be referred to as membersof the set. Objects of a set can be the same or different. In someinstances, objects of a set can share one or more common properties.

As used herein, the term “adjacent” refers to being near or adjoining.Adjacent objects can be spaced apart from one another or can be inactual or direct contact with one another. In some instances, adjacentobjects can be coupled to one another or can be formed integrally withone another.

As used herein, the terms “couple,” “coupled,” and “coupling” refer toan operational connection or linking. Coupled objects can be directlyconnected to one another or can be indirectly connected to one another,such as via an intermediary set of objects.

As used herein, the terms “substantially” and “substantial” refer to aconsiderable degree or extent. When used in conjunction with an event orcircumstance, the terms can refer to instances in which the event orcircumstance occurs precisely as well as instances in which the event orcircumstance occurs to a close approximation, such as accounting fortypical tolerance levels or variability of the embodiments describedherein.

As used herein, the terms “optional” and “optionally” mean that thesubsequently described event or circumstance may or may not occur andthat the description includes instances where the event or circumstanceoccurs and instances in which it does not.

As used herein, the term “size” refers to a characteristic dimension ofan object. Thus, for example, a size of an object that is spherical canrefer to a diameter of the object. In the case of an object that isnon-spherical, a size of the non-spherical object can refer to adiameter of a corresponding spherical object, where the correspondingspherical object exhibits or has a particular set of derivable ormeasurable properties that are substantially the same as those of thenon-spherical object. Thus, for example, a size of a non-sphericalobject can refer to a diameter of a corresponding spherical object thatexhibits light scattering or other properties that are substantially thesame as those of the non-spherical object. Alternatively, or inconjunction, a size of a non-spherical object can refer to an average ofvarious orthogonal dimensions of the object. Thus, for example, a sizeof an object that is a spheroidal can refer to an average of a majoraxis and a minor axis of the object. When referring to a set of objectsas having a particular size, it is contemplated that the objects canhave a distribution of sizes around the particular size. Thus, as usedherein, a size of a set of objects can refer to a typical size of adistribution of sizes, such as an average size, a median size, or a peaksize.

As used herein, the term “electrolyte” means an ion-conducting, butelectronically insulating medium into which the electrodes of anelectrochemical cell are disposed. A liquid electrolyte contains one ormore liquid solvents and one or more salts that readily disassociatewhen dissolved in these solvents. Liquid electrolytes may also containadditives that enhance a performance characteristic of theelectrochemical cell into which the electrolyte is disposed.

As used herein, the term “battery” means a rechargeable electrochemicaldevice that converts stored chemical energy into electrical energy,including voltaic cells that may each include two half-cells joinedtogether by one or more conductive liquid electrolytes.

As used herein, in the context of a cosolvent solution and a majority orprimary solvent of such cosolvent solution, the term “majority” or“primary” means, for a two solvent cosolvent solution, a solvent having50% or greater volume of the total solvent volume (% vol./vol.), or 50%or greater weight of the total solvent weight (% weight/weight). For acosolvent solution having three or more solvents, the majority/primarysolvent is the solvent present in the greatest quantity (by volume orweight) as compared to the quantities of any of the other solvents ofthe cosolvent solution. These determinations are preferably made beforeaccounting for any salt or additive to the cosolvent solution. A“minority” or “secondary” solvent in a cosolvent solution is any othersolvent other than the majority/primary solvent. For purposes of thispresent invention when considering cosolvent solutions, water is never amajority solvent and may be a minority/secondary solvent. Water ispurposefully present as minority solvent in greater quantity than wouldbe incidental or present as a contaminant having 2% or greater volume ofthe total solvent volume (% vol./vol.), or 2% or greater weight of thetotal solvent weight (% weight/weight). An aqueous electrolyte includeswater as a majority solvent when in a cosolvent electrolyte and in someinstances water may be the only solvent present in a single-solventelectrolyte. Cosolvent water, with water as a significant (e.g., about2% or greater) solvent but not a majority solvent, may produce anelectrolyte that is sometimes referred to as quasi-aqueous to indicatethat water is present in more than trace amounts but is not the majoritysolvent for two or more cosolvents.

As used herein, the term “variable potential material” means a material,that when used as an electrode in an electrochemical device, experiencesa variable potential as a function of state of charge. Transition metalcyanide coordination compound (TMCCC) materials are an example of avariable potential material. Other examples include: transition metaloxides including but not limited to lithium cobalt oxide, lithium nickeloxide, lithium manganese oxide, lithium nickel manganese cobalt oxide,lithium nickel cobalt aluminum oxide, manganese dioxide, sodiummanganese oxide, sodium cobalt oxide, and tungsten trioxide; sulfur,lithium sulfide; carbons including but not limited to graphite,mesoporous carbons, and activated carbons including charcoal; siliconincluding nanostructured silicon; polymers including but not limited topolypyrrole, polythiophene, polyanilene, andpoly(3,4-ethylenedioxythiophene) polystyrene sulfonate; and combinationsof one or more of the above.

As used herein, the term “additive” in the context of a compound,substance, material, mixture, blend, composition, mix, amalgamation, orother addition or assembly relative to an electrochemical deviceincluding an electrolyte in chemical communication to a variablepotential material that is capable of undergoing an electrochemicalredox reaction with at least one electrode of the electrochemicaldevice. One or more additives may be used in the electrochemical device.In some cases, this electrochemical redox reaction may be irreversible,resulting in consumption of the additive. In other cases, that reactionmay be reversible resulting in non-consumption of the additive, orconversion of the additive through intermediate reactions of theadditive, allowing the additive to be recycled and reused, such asthrough chemical recycling. In some embodiments, the additive may beadded to the electrolyte before the electrolyte is added to the cell. Insome embodiments, the additive may be added to the electrodes beforethey are added to the cell. In some embodiments, the additive may beadded to the slurry or paste used to produce the electrodes.

Electrode Materials

Some disclosed embodiments of the invention relate to battery electrodematerials in which dimensional changes in a host crystal structureduring charging and discharging are small, thereby affording long cyclelife and other desirable properties. Such dimensional changes canotherwise result in mechanical deformation and energy loss, as evidencedby hysteresis in battery charge/discharge curves.

Some embodiments relate to a class of transition metal cyanidecoordination compound (TMCCC) electrode materials having stiff openframework structures into which hydrated cations can be reversibly andrapidly intercalated from aqueous (e.g., majority water-based)electrolytes or other types of electrolytes. In particular, TMCCCmaterials having the Prussian Blue-type crystal structure affordadvantages including greater durability and faster kinetics whencompared to other intercalation and displacement electrode materials. Ageneral formula for the TMCCC class of materials is given by:

A_(x)P_(y)[R(CN)_(6-j)L_(j)]_(z) .nH₂O, where:

A is a monovalent cation such as Na⁺, K⁺, Li⁺, or NH₄ ⁺, or a divalentcation such as Mg²⁺ or Ca²⁺;P is a transition metal cation such as Ti³⁺, Ti⁴⁺, V²⁺, V³⁺, Cr²⁺, Cr³⁺,Mn⁺, Mn²⁺, Mn³⁺, Fe²⁺, Fe³⁺, Co²⁺, Co³⁺, Ni²⁺, Cu⁺, Cu²⁺, or Zn²⁺, oranother metal cation such as Al³⁺, Sn²⁺, In³⁺, or Pb²⁺;R is a transition metal cation such as V²⁺, V³⁺, Cr²⁺, Cr³⁺, Mn⁺, Mn²⁺,Mn³⁺, Fe²⁺, Fe³⁺, Co²⁺, Co³⁺, Ru²⁺, Ru³⁺, Os²⁺, Os³⁺, Ir²⁺, Ir³⁺, Pt²⁺,or Pt³⁺;L is a ligand that may be substituted in the place of a CN⁻ ligand,including CO (carbonyl), NO (nitrosyl), or Cl⁻;0≤x≤2;0<y≤4;0<z≤1;0≤j≤6; and0≤n≤5.

Figures

FIG. 1 illustrates a schematic of batteries using the higher and loweranode reactions for the MnHCMn anode and the reaction potential of theCuHCF cathode. This schematic shows the operational modes of a batterycontaining a TMCCC cathode and a TMCCC anode used together in twodifferent electrolytes; 1) an aqueous electrolyte, and 2) a cosolventelectrolyte. In the aqueous electrolyte, rapid hydrogen evolution occursabove the lower operational potential of the anode, so only the upperoperational potential of the anode can be used. The result is a 0.9 Vcell. But, in the cosolvent electrolyte, hydrogen production issuppressed, resulting in efficient use of the lower operationalpotential of the anode and a full cell voltage of 1.7 V.

FIG. 2 illustrates a unit cell of the cubic Prussian Blue crystalstructure, one example of a TMCCC structure. Transition metal cationsare linked in a face-centered cubic framework by cyanide bridgingligands. The large, interstitial A sites can contain water or insertedalkali ions.

FIG. 3 illustrates a cyclic voltammogram of MnHCMn in cosolvents. Cyclicvoltammetry of the lower operational potential of manganesehexacyanomanganate(II/I) is shown in aqueous 1 M NaClO₄ and 1 M NaClO₄containing various concentrations of acetonitrile. The position andhysteresis between the current peaks vary only slightly withacetonitrile concentration, indicating that the reaction mechanism andperformance is largely independent of the cosolvent.

FIG. 4 illustrates a cyclic voltammogram of MnHCMn in cosolvents. Cyclicvoltammetry of the lower operational potential of manganesehexacyanomanganate(II/I) is shown in aqueous 1 M NaClO₄ and 1 M NaClO₄containing 95% solvent volume acetonitrile and 5% solvent volume water.Reversible cycling is achieved even with only 5% water present. Thebackground current at −0.9 V is 1 mA in purely aqueous electrolyte, butonly 0.1 mA in the primarily organic cosolvent electrolytes,demonstrating improved coulombic efficiency with an organic primarycosolvent.

FIG. 5 illustrates a cyclic voltammogram of MnHCMn in cosolvents. Cyclicvoltammetry of the lower operational potential of manganesehexacyanomanganate(II/I) is shown 1 M NaClO₄ containing 5% solventvolume water, 47.5% solvent volume acetonitrile, and 47.5% solventvolume of one of sulfolane, propylene glycol monoethyl ether,hydroxypropionitrile, or gamma-valerolactone. In all cases, cycling ofMnHCMn is shown to be reversible.

FIG. 6 illustrates a cyclic voltammogram of MnHCMn in cosolvents. Cyclicvoltammetry of the lower operational potential of manganesehexacyanomanganate(II/I) is shown 1 M NaClO₄ containing 5% solventvolume water, 47.5% solvent volume acetonitrile, and 47.5% solventvolume of one of ethylene carbonate, dimethyl carbonate, or1,3-dioxolane, or containing 5% solvent volume water, 10% solvent volumeacetonitrile, and 85% solvent volume propylene carbonate. In all cases,cycling of MnHCMn is shown to be reversible.

FIG. 7 illustrates a cyclic voltammogram and integrated current ofMnHCMn in 1 M NaClO₄ in 90% solvent volume acetonitrile and 10% solventvolume water. Main Figure: cyclic voltammetry of MnHCMn(II/I) in 1 MNaClO₄, 90%/10% MeCN/H₂O shows an extremely reversible reaction centeredat −0.75 V vs. SHE. The open circuit potential of the material is abovethe upper reaction [MnHCMN(III/II)] so during the first reductive sweeptwo reactions are observed. The peak current of ±1.2 A/g is theequivalent of a 20C galvanostatic cycling rate, indicating extremelyfast kinetics. Inset Figure: integration of the current during each scangives the specific charge and discharge capacity of the electrode. About57 mAh/g is observed, in close agreement with the approximatetheoretical specific capacity of 60 mAh/g. A coulombic efficiency ofwell over 95% is achieved. There is little capacity fading, in agreementwith GCPL measurements of MnHCMn(II/I) in the same electrolyte.

FIG. 8 illustrates a cyclic voltammogram of CuHCF in cosolventscontaining varying amounts of acetone. Cyclic voltammetry is shown ofthe copper hexacyanoferrate cathode in aqueous 1 M NaClO₄ and in 1 MNaClO₄ containing up to 90% solvent volume acetone and as little as 10%solvent volume water. There is little change in the potential of thereaction with increasing amounts of the cosolvent. No clear trend isobserved in the small effects of the cosolvent on the reaction potentialand kinetics of the charge and discharge of the electrode.

FIG. 9 illustrates a cyclic voltammogram of MnHCMn in 90% or 100% MeCN.Cyclic voltammetry is shown of the lower reaction manganesehexacyanomanganate(II/I) in 1 M NaClO₄ containing either 100% solventvolume acetonitrile or 90% solvent volume acetonitrile and 10% solventvolume water. The electrode has very poor kinetics and a poor currentresponse in the 100% solvent volume acetonitrile electrolyte. Incontrast, the addition of 10% water to the acetonitrile results in areaction with faster kinetics and a higher peak current.

FIG. 10 illustrates a cycle life of MnHCMn in half cells. During cyclingin 1 M NaClO₄ containing 90% solvent volume acetonitrile and 10% solventvolume water, MnHCMn(II/I) shows good cycle life, losing only 5% of itsinitial discharge capacity after 15 cycles. In contrast, in aqueous 1 MNaClO₄ with no acetonitrile present, 25% of the initial dischargecapacity is lost after 15 cycles.

FIG. 11 illustrates a set of potential profiles of MnHCMn in half cells.The potential profiles of MnHCMn(II/I) are shown during cycling in twodifferent electrolytes: aqueous 1 M NaClO₄ containing no organiccosolvent, and 1 M NaClO₄ containing 90% solvent volume acetonitrile and10% solvent volume water. In both electrolytes, the MnHCMn reaction iscentered at −0.95 V vs. Ag/AgCl, or equivalently, −0.75 V vs. SHE.Though both samples were cycled at the same 1C rate, the sample operatedin the purely aqueous electrolyte shows a much lower capacity of 40mAh/g as rapid hydrolysis upon its insertion into the cell consumed onethird of its capacity. In contrast, the MnHCMn electrode operated in theelectrolyte containing the organic primary cosolvent had a specificdischarge capacity of over 55 mAh/g, much closer to the maximumtheoretical value (see FIG. 10).

FIG. 12 illustrates a set of coulombic efficiencies of MnHCMn in halfcells operated by galvanostatic cycling between −0.95 V and −0.5 V vs.SHE. The coulombic efficiency is defined as the ratio for each cycle ofthe discharge capacity divided by the charge capacity. In the cellcontaining an electrolyte of 1 M NaClO₄ and 100% solvent volume water, acoulombic efficiency of less than 99% is observed. In three identicalcells each containing an electrolyte of 1.4 M NaClO₄, 95% solvent volumeacetonitrile, and 5% solvent volume water, a coulombic efficiency ofover 99.5% is observed.

FIG. 13 illustrates a cycle life of CuHCF in half cells. During cyclingof CuHCF at a 1C rate in aqueous 1 M NaClO₄ containing no organiccosolvents, 4% of the initial discharge capacity is lost after 50cycles. In contrast, during cycling of CuHCF at a 1C rate in 1 M NaClO₄containing 90% solvent volume acetonitrile and 10% solvent volume water,zero capacity loss is observed after 300 cycles.

FIG. 14 illustrates a set of GCPL vs. time profiles of MnHCMn vs. CuHCFin the full cell. The potential profiles of the CuHCF cathode andMnHCMn(II/I) anode in a full cell, and the full cell voltage profile areshown. The electrolyte was 1 M NaClO₄, 10% solvent volume H₂O, 90%solvent volume MeCN, and cycling was performed at a 1C rate with theanode operated as the working electrode. An excess of CuHCF was used inthis case to avoid any oxygen generation at high potentials, so thepotential profile of the cathode is flatter than that of the anode.

FIG. 15 illustrates a full cell voltage profile. The full cell voltageprofile is of the cell shown in FIG. 13. The average voltage of the cellis 1.7 V, nearly double the voltage achievable if the MnHCMn(III/II)reaction is used. The result is a cell with significantly higher energyand power.

FIG. 16 illustrates a representative secondary electrochemical cell 1600schematic having one or more TMCCC electrodes disposed in contact with acosolvent electrolyte as described herein. Cell 1600 includes a negativeelectrode 1605, a positive electrode 1610 and an electrolyte 1615electrically communicated to the electrodes.

Overview

A battery (or cell) comprises an anode, a cathode, and an electrolytethat is in contact with both the anode and the cathode. Both the cathodeand the anode contain an electrochemically active material that mayundergo a change in valence state, accompanied by the acceptance orrelease of cations and electrons. For example, during discharge of abattery, electrons are extracted from the anode to an external circuit,while cations are removed from the anode into the electrolyte.Simultaneously, electrons from the external circuit enter the cathode,as do cations from the electrolyte. The difference in theelectrochemical potentials of the cathode and anode results in a fullcell voltage. This voltage difference allows energy to be extracted fromthe battery during discharge, or stored in the battery during charge.

The battery may be rechargeable and include electrodes that may be madeof variable potential material. Further, the battery may include one ormore additives in chemical communication with the electrolyte. Theadditive(s) participate in one or more limited side-reactions with oneor more of the electrodes. These limited side-reactions degrade chargingof its associated electrode(s) for the duration of the side-reaction.This allows other electrodes to begin charging immediately at fullcoulombic efficiency. Consequently, in response to a charging source,the electrodes may have unbalanced charges. However, with appropriateselection and coordination of the limited side-reaction(s), the overallenergy density of the electrochemical device may be greater than thecase without the limited side-reactions.

For example, an electrochemical device may include a rechargeablebattery including two variable potential electrodes having the samenominal charge capacities and linear variation in their electrochemicalpotentials with their states of charge, the first of which (cathode)undergoes a full charge or discharge between 1.0 volts and 1.5 volts asmeasured with respect to an arbitrary reference electrode, and thesecond of which (anode) undergoes a full charge or discharge between−1.0 volts and −1.5 volts with respect to the same reference electrode,in an electrolyte. Nominally, the cell may reach a full charge at 3.0volts, with the cathode reaching a potential of 1.5 volts at full chargeand the anode reaching a potential of −1.5 volts at full charge, withrespect to the reference electrode. During a discharge of this cell to2.0 volts, the cathode may discharge fully to a potential of 1.0 voltsand the anode may discharge fully to a potential of −1.0 volts withrespect to the reference electrode. However, in some cases there may belimitations to the cell, or with the chemistry of the cell, among avariety of other different reasons, that the cell cannot charge to 3.0volts, but rather to a lower voltage. An example of this limitation isan electrolyte that is only electrochemically stable over a 2.8 voltrange, less than a 3.0 volt range required to fully charge bothelectrodes. An embodiment of the present invention may address thesesituations. Various cases are described herein, for example, see thediscussion below with respect to FIG. 31.

The electrolyte in a battery allows ions to flow from one electrode tothe other, but that insulates the two electrodes from one anotherelectronically. Typically battery electrolytes include aqueous acids andsalts in lead acid and bases nickel/metal hydride batteries, and organicliquids containing lithium salts in lithium-ion batteries. Theelectrolyte may also contain additives that stabilize the electrodes,prevent side chemical reactions, or otherwise enhance batteryperformance and durability. The electrolyte may also contain multipleliquid components, in which case they are known as cosolvents. Theliquid component making up the majority of the electrolyte is typicallyknown as the primary solvent, while those making up the minority areknown as minority solvents.

Organic cosolvents have been used in battery electrolytes in some typesof batteries. For example, commercial lithium-ion battery electrolytescontain a variety of organic cosolvents, including ethylene carbonate,diethyl carbonate, propylene carbonate, and others. Those batteryelectrodes never include water as a minority solvent. Other aqueouselectrolyte batteries such as lead acid, nickel/metal hydride, and flowbatteries typically do not use cosolvent electrolytes. There is noprecedent among previously documented battery systems for cosolventelectrolytes containing water as a minority component.

An electrolyte containing organic cosolvents in combination with wateras a minority cosolvent offers several advantages in comparison toelectrolytes that are either primarily aqueous or that contain solelyorganic cosolvents. First, when water is present as only a minoritycosolvent, its decomposition into hydrogen and oxygen is suppressed, anda larger practical electrochemical stability window is achieved (FIGS.1, 4). Second, electrode materials and other battery materials that arewater-sensitive and may decompose by a hydrolysis mechanism are morestable when water is only a minority component of the system. Third,water has higher ionic conductivity than the organic solvents typicallyused in battery electrodes, so its presence as a minority cosolventincreases the electrolyte conductivity.

Cosolvent electrolytes are of interest for the stabilization of TMCCCelectrodes that have inherent solubility in aqueous batteryelectrolytes. Copper hexacyanoferrate (CuHCF) is a TMCCC recentlydemonstrated to be a high performance battery electrode. In the openframework structure of CuHCF, iron is six-fold, octahedrally coordinatedto the carbon ends of the cyanide branching ligands, while copper isoctahedrally nitrogen-coordinated (FIG. 3). Depending on the method ofsynthesis, the A sites in CuHCF may contain potassium or another alkalication such as sodium or lithium, or another type of cation such asammonium. More generally, for a TMCCC of the general chemical formulaA_(x)P_(y)[R(CN)₆]_(z).nH₂O, alkali cations A⁺ and water occupy theinterstitial A Sites, transition metal P cations are six-fold nitrogencoordinated, and transition metal R cations are six-fold carboncoordinated.

In the work described here, the electrochemical cells contained a TMCCCworking electrode, a counter electrode, an electrolyte in contact withboth the anode and cathode, and an Ag/AgCl reference electrode used toindependently measure the potentials of the anode and cathode duringcharge and discharge of the cell. When the electrode of interest was acathode material, then the working electrode was the cathode, and thecounter electrode was the anode. When the electrode of interest was ananode material, then the working electrode was the anode, and thecounter electrode was the cathode. In the case that the cell did notcontain both a TMCCC cathode and a TMCCC anode, a capacitive activatedcharcoal counter electrode was used to complete the circuit whileallowing the study of a single TMCCC electrode.

Electrochemical characterization of electrodes was performed usingcyclic voltammetry (CV) and galvanostatic cycling with potentiallimitation (GCPL). During the CV technique, the potential of the workingelectrode is swept at a constant rate between high and low cutoffpotentials, and the resulting current into or out of the electrode ismeasured. During the GCPL technique a constant current is applied to thecell until the working electrode reaches a maximum or minimum potential;upon reaching this potential extreme, the sign of the current isreversed.

Researchers have used TMCCCs as battery electrodes in cells containingaqueous and organic electrolytes For example, the reversible reductionof Prussian Blue to Everitt's Salt has allowed its use as an anode inaqueous cells. However, the electrochemical potential of Prussian Blueis relatively high, so using it as an anode with a TMCCC cathode resultsin a low full cell voltage of 0.5-0.7 V vs. SHE. Such low voltages makethese cells impractical, as many cells in series would be required toachieve the high voltages needed for many applications.

Chromium hexacyanochromate (CrHCCr) has also been used as an anode infull cells that also contained Prussian Blue cathodes, and anaqueous/Nafion electrolyte. The performance of these cells was limitedby the low potential and poor coulombic efficiency of CrHCCr in aqueouselectrolytes and the use of acidic electrolytes in which CrHCCrhydrolyzes.

TMCCC anodes containing electrochemically active hexacyanomanganategroups have also been recently demonstrated. Examples include manganesehexacyanomanganate (MnHCMn), and zinc hexacyanomanganate (ZnHCMn). Inhexacyanomanganate-based TMCCC anodes, the hexacyanomanganate groupsundergo two electrochemical reactions. First, Mn^(III)(CN)₆ can bereversibly reduced to Mn^(II)(CN)₆ at potentials near or above 0 V vs.SHE. Second, Mn^(II)(CN)₆ can be reduced to Mn^(I)(CN)₆ at lowerpotentials, typically below −0.4 V vs. SHE. In general, the lowerreaction cannot be efficiently used in aqueous electrolytes due to thesimultaneous generation of hydrogen gas at such low potentials. Oneexception is chromium hexacyanomanganate (CrHCMn), which has a lowerreaction potential of about −0.35 V, but high-purity CrHCMn isextraordinarily difficult to synthesize due to its affinity to formother phases such as mixed cyanides and oxides of chromium. In no priorart has the lower reaction of any hexacyanomanganate-based TMCCC beenused with high coulombic efficiency in aqueous electrolytes.

Though the use of a basic electrolyte would result in a lower potentialfor the onset of H₂ generation, TMCCCs rapidly decompose at high pHexcept in the presence of an excess of free cyanide anions, which are asevere safety hazard. Mildly acidic or neutral electrolytes are neededfor them to be stable. Thus, only the upper reaction of MnHCMn can beused without deleterious H₂ production. As the upper stability limit ofthese aqueous electrolytes is near 1 V, MnHCMn can be paired with acathode such as CuHCF to produce a battery with an average full cellvoltage of about 0.9-1 V.

TMCCCs have also been used as cathodes, but not as anodes, in organicelectrolyte batteries. Most commonly, they have been used as cathodes inplace of the standard LiCoO₂ cathode found in high-voltage organicelectrolyte Li-ion cells. A number of studies have demonstrated TMCCCscontaining electrochemically active iron and/or manganese as cathodes inthese high voltage cells.

TMCCCs have not been previously used as battery electrodes in cosolventelectrolytes in which water is a minority cosolvent. In recentlypublished patent application, we described the opportunity to do so forthe specialized case of water acting as the primary cosolvent. However,a practical cosolvent had not yet been identified, and the cosolventelectrolytes described in that document decompose into multiple phasesunder some circumstances, making them impractical for use in an actualbattery. In addition, in that previous work, the idea of a cosolvent wasdescribed and claimed in the context of an organic liquid additive to awater, with water as the primary solvent of the electrolyte. Herein wedescribe for the first time the principles for selecting cosolvents andelectrolyte salts to combine with water to produce stable, single phaseaqueous cosolvent electrolytes in which TMCCC electrodes operate withhigh efficiency, fast kinetics, and long lifetime. In addition, wedemonstrate for the first time the operation of TMCCC electrodes inaqueous cosolvent electrolytes in which water is a minority solvent ofthe electrolyte, and an organic solvent is the primary solvent.

U.S. Patent Application No. 61/722,049 filed 2 Nov. 2012 includes adiscussion of various electrolyte additives to aqueous electrolytes, aswell as coatings on the electrodes of electrochemical cells, that canimprove a rate of capacity loss. U.S. Patent Application No. 61/760,402filed 4 Feb. 2013 includes a discussion of a practical TMCCC anode. Bothof these patent applications are hereby expressly incorporated in theirentireties by reference thereto for all purposes.

Herein we discuss and demonstrate for the first time the use of apractical aqueous cosolvent electrolyte for batteries containing a TMCCCanode and a TMCCC cathode. The use of an organic liquid as the primarysolvent, with water as a minority solvent has no significant effects onthe kinetics or reaction potentials of either TMCCC anodes or TMCCCcathodes as compared to the performance of those electrolytes in aqueouselectrolytes containing no organic solvents. In addition, the cosolventstabilizes TMCCCs against dissolution and hydrolysis, resulting ingreater electrode stability and longer cycle and calendar life.

Our previous demonstration of the operation of TMCCC cathodes incosolvent electrolytes did not demonstrate the use of an organic solventas the majority electrolyte, and it considered only the effect of anorganic minority cosolvent on the performance of TMCCC cathodes withoutshowing the reduction to practice of a full cell containing a cosolventelectrolyte. Furthermore, it did not address the extreme sensitivity ofhexacyanomanganate-based TMCCC anodes to electrolyte composition. Forthese reasons, among others, the work described here is novel andindependent.

The addition of an organic cosolvent as the majority component to thebattery electrolyte is especially important for the performance andlifetime of TMCCC anodes. Whereas without any cosolvents, the upperreaction of the MnHCMn anode must be used in aqueous electrolytes, herewe show that the addition of a cosolvent to the electrolyte suppresseselectrolysis of water to hydrogen gas. In a full cell also containing aCuHCF cathode, the result in an increase in average discharge voltagefrom about 0.9 V to about 1.7 V (FIG. 1). Nearly doubling the cellvoltage has extraordinary ramifications for the performance and cost ofthe battery. Energy scales proportionally with voltage, while powerscales with the square of the voltage. Thus, nearly doubling the voltagewhile using the same electrode materials results in about twice theenergy and nearly four times the power, at about the same materialscost. Without the presence of a cosolvent that limits the rate ofhydrogen production at the anode, cells with a TMCCC anode and cathodecannot achieve high efficiency at voltages above about 1.3 V. Thus, theaddition of the cosolvent increases the maximum practical voltage,energy, and power of the cell.

Prior study of TMCCCs in organic electrolytes did include the use oforganic cosolvent electrolytes in some cases. However, in anhydrousconditions, the kinetics of TMCCC electrodes are vastly reduced, makingthese electrodes impractical for high power applications. In this work,we demonstrate for the first time the use of aqueous cosolventelectrolytes containing non-negligible amounts of water. That water mustbe present for the TMCCC electrodes to be rapidly charged or discharged.

As a first example, acetonitrile (also known as methyl cyanide, or MeCN)is chosen as a cosolvent to be used in electrolytes for batteriescontaining TMCCC electrodes. MeCN is fully miscible with water and iselectrochemically stable over a much wider potential range than wateritself. High purity, anhydrous MeCN is used in commercialultracapacitors. Here, reagent-grade MeCN was used, as low voltage cellsare less sensitive to electrolyte impurities that may result inparasitic side reactions at extreme potentials.

The choice of MeCN provides an additional benefit for the specific caseof a battery containing TMCCC electrodes. In a cosolvent electrolytecontaining MeCN as the primary solvent, the solvation shells of theTMCCC electrode particles will primarily be cyanide groups in whichnitrogen faces the particle. This completes the six-fold nitrogencoordination of P-site cations in the particle at the surface oradjacent to hexacyanometalate vacancies. The result is improved materialstability via suppression of dissolution via the formation of ahydration shell.

Other examples of organic solvents include ethylene carbonate, propylenecarbonate, and dimethyl carbonate; sulfolane; 1,3 dioxolane; propyleneglycol monoethyl ether; hydroxypropionitrile; diethylene glycol;gamma-valerolactone; acetone; ethylene glycol and glycerol. Organiccosolvents must be polar to allow them to form miscible single phasesolutions with water and a salt, but they may be either protic oraprotic.

It is desirable when using hexacyanomanganate-based TMCCC anodes to usewater as only a minority cosolvent, and organic liquids as the primarycosolvents. The manganese-carbon bond in hexacyanomanganate is labileand cyanide can be replaced by water and/or hydroxide. The choice of alarger, less polar organic species as the primary solvent results inweaker bonding to Mn and steric hindrance, both of which protect thehexacyanomananate group from suffering ligand exchange leading to itsdecomposition.

Proper selection of the electrolyte cosolvents, salts, and anyadditional additives will result in a single-phase system in which allof the components are miscible and do not phase segregate. Phasesegregation in a battery electrolyte is undesirable because iontransport will occur primarily in the phase containing the higher saltconcentration, while the other, less conductive phase or phases willimpede the transport of ions. It is not enough to simply choose liquidsthat are miscible, as the addition of a salt can lead to decompositionof the electrolyte into multiple phases: for example, one that is mostlywater, that has a high salt concentration, and that contains a smallamount of the organic solvent, and a second phase that is mostly organicsolvent, and contains little water or salt leads to poor performancewhen there is phase segregation, a problem addressed by proper selectionof electrolyte cosolvents.

A very limited number of common electrolyte salts that are highly watersoluble are also appreciably soluble in organic solvents. This isbecause most organic solvents have dielectric constants much lower thanthat of water. In other words, organic solvents are typically not aspolar as water, so the formation of a solvation shell during thedissolution of an ionic salt is not energetically favorable. Forexample, potassium nitrate, which has a saturation of 3.6 M in water atroom temperature, is only sparingly soluble in most organic solvents.

Here, to demonstrate a reduction to practice of the operation of TMCCCelectrodes in cosolvent electrolytes containing an organic primarysolvent, an embodiment may use sodium perchlorate hydrate as theelectrolyte salt in cosolvent electrolytes of water and MeCN. The choiceof NaClO₄.H₂O is based on its ability to dissolve in high concentrations(greater than 1 M) over the entire range of cosolvent ratios from100%/0% water/MeCN to 0%/100% water/MeCN without forming biphasicsystems.

The ternary phase diagrams describing the solubility of salts such asNaClO₄ in cosolvents such as water/MeCN are tabulated. The general needfor high salt concentration and a monophasic electrolyte can be used toselect other combinations of salts and cosolvents from these data.

Other cosolvents besides acetonitrile that can be used with water inelectrolytes for use in batteries containing TMCCC anodes include, butare not limited to, methanol, ethanol, isopropanol, ethylene glycol,propylene glycol, glycerine, tetrahydrofuran, dimethylformamide, andother small, polar linear and cyclic alcohols, polyols, ethers, andamines. However, while many of these solvents are fully miscible withpure water, they are not miscible in the presence of concentrated salt.For example, more than a few percent isopropyl alcohol willphase-segregate from concentrated aqueous salts of sodium, which thiswill not occur if acetonitrile is used in the place of isopropylalcohol. A proper selection of the cosolvents and the salt will resultin a single-phase solution.

CuHCF was synthesized as reported previously. An aqueous solution ofCu(NO₃)₂, and a second aqueous solution of K₃Fe(CN)₆ were added to waterby simultaneous, dropwise addition while stirring. The finalconcentrations of the precursors were 40 mM Cu(NO₃)₂ and 20 mMK₃Fe(CN)₆. A solid, brown precipitate formed immediately. It wasfiltered or centrifuged, washed, and dried. In a prior study, CuHCFsynthesized by this method was found to have the compositionK_(0.7)Cu[Fe(CN)₆]_(0.7).2.8H₂O. The CuHCF was found to have the cubicPrussian Blue open framework crystal structure using X-ray diffraction(XRD). The CuHCF was composed of nanoparticles about 50 nm in size, asverified by scanning electron microscope (SEM).

MnHCMn was produced state by adding a 10 mL aqueous solution containing0.0092 mmol KCN to a 10 mL aqueous solution containing 0.004 mmolMnCl₂.4H₂O under constant stirring in the dark in a nitrogen atmosphere.After stirring the solution for 20 minutes, the resulting dark greenprecipitate was centrifuged, washed with methanol, and dried at roomtemperature in a nitrogen atmosphere. Analysis of this material usingX-ray diffraction showed that it had the monoclinic crystal structurecharacteristic of MnHCMn(II) synthesized by a similar method Compositionanalysis using inductively coupled plasma optical emission spectrometry(ICP-OES) revealed that this material was K_(0.4)Mn[Mn(CN)₆]_(0.6).nH₂O(0<n<4).

Aqueous cosolvent electrolytes were prepared from reagent-gradeNaClO₄.H₂O, de-ionized water, and reagent grade MeCN. All electrolyteswere pH-neutral, but not buffered. The salt was dissolved in aconcentration of 1 M in cosolvents with solvent volume ratios of100%/0%, 90%/10%, 50%/50%, 10%/90%, and 0%/100% water/MeCN.

Electrodes containing the freshly synthesized TMCCCs were prepared asreported previously. The electrochemically active material, carbonblack, and polyvinylidene difluoride (PVDF) binder were ground by handuntil homogeneous, and then stirred in 1-methyl-2-pyrrolidinone (NMP)solvent for several hours. This slurry was deposited on anelectronically conductive carbon cloth substrate using a doctor blade orspatula. Other substrates including foils and meshes of stainless steeland aluminum can also be used. These electrodes were dried in vacuum at60° C. For practical batteries, the binder is preferably selected suchthat it is stable against dissolution or excessive swelling in thecosolvent electrolyte, but is still fully wetted by the cosolvent.Methods for determining binder/electrolyte compatibilities such asHansen Solubility Parameter analysis are well known.

Activated charcoal counter electrodes were prepared by grinding thecharcoal with PVDF before stirring in NMP for several hours, followed bydeposition and drying on conductive substrates following the sameprocedure as in the case of electrodes containing a TMCCC.

Electrochemical Characterization

Half-cell measurements were performed on TMCCC electrodes in cosolventelectrolytes. The cell contained the working electrode, an Ag/AgClreference electrode, an activated charcoal counter electrode, and thedeaerated electrolyte. Cyclic voltammetry was performed on the workingelectrode.

Example 1

A MnHCMn electrode was disposed in a half cell in the configurationdescribed above and operated by cyclic voltammetry. The reactionpotentials of the reactions of MnHCMn with 1 M Na⁺ were found to beabout −0.76 V and 0.04 V vs. SHE. The potential of the lower reaction ofMnHCMn varied only slightly with the addition of MeCN to theelectrolyte, from 0% MeCN to 95% MeCN (FIG. 3-4). The magnitude and signof the small shift in reaction potential showed no trend with MeCNconcentration (FIG. 3).). Furthermore, MnHCMn was found to cyclereversibly in 95% MeCN at with Na⁺ salt concentrations of both 1 M and1.4 M.

Example 2

A MnHCMn electrode was disposed in a half cell in the configurationdescribed above and operated by cyclic voltammetry. MnHCMn was found tocycle reversibly in cosolvent electrolytes containing water as aminority cosolvent comprising 5% of the total solvent volume, and withequal quantities of MeCN and a second organic cosolvent comprising theremaining 95% of the total solvent volume (FIG. 5-6). These secondorganic cosolvents were one of: sulfolane, propylene glycol monoethylether, hydroxypropionitrile, gamma-valerolactone, ethylene carbonate,dimethyl carbonate, and 1,3-dioxolane. In these example electrolytes,the solvent volume of MeCN is as little as 10%, with another primaryorganic cosolvent such as propylene carbonate comprising 85% solventvolume. These electrolyte compositions of matter demonstrate the use ofmultiple organic cosolvents in combination with water as a minoritycosolvent.

Example 3

A MnHCMn electrode was disposed in a half cell in the configurationdescribed above and operated by cyclic voltammetry. Over 55 mAh/g ofspecific discharge capacity was achieved for the lower reaction ofMnHCMn in a cosolvent electrolyte of 1 M NaClO₄ in 90% MeCN and 10%water (FIG. 7). This is comparable to the 50-60 mAh/g capacitiestypically achieved for the upper reaction of MnHCMn at 0.05 V in aqueouselectrolytes. With no loss in specific capacity of the anode, but a gainin full cell voltage of about 0.8 V, full cells that operate by usingthe lower reaction of MnHCMn will have nearly double the energy of thosethat operate by using the upper reaction of MnHCMn, with the sameelectrode materials (and associated costs). This makes the use of thelower reaction, and therefore, the use of a cosolvent electrolyte,critically important to the economics and viability of the battery.

Example 4

A CuHCF electrode was disposed in a half cell in the configurationdescribed above and operated by cyclic voltammetry. The half cellscontained electrolytes of 1 M NaClO₄ and quantities of water and acetoneup to 90% acetone. During cyclic voltammetry the reaction potential ofCuHCF with 1 M Na⁺ was observed to be centered at 0.84 V vs. SHE, whichis consistent with the previously observed value (FIG. 8). The reactionpotential and peak current hysteresis of CuHCF during CV varied onlyslightly, 1 M NaClO₄ cosolvents containing increasing amounts of acetoneup to 90% of the total solvent volume.

Example 5

A MnHCMn electrode was disposed in a half cell in the configurationdescribed above and operated by cyclic voltammetry. In this example, thehalf cell contained an electrolyte of pure MeCN and no water, and 1 MNaClO4. A much lower peak current of MnHCMn was observed in MeCNelectrolyte without water added as a minority cosolvent (FIG. 9). Incontrast, the CV curves shown in FIG. 3, FIG. 4, and FIG. 8 show thatthere is little change in the voltage difference between the peakcurrents in oxidation and reduction. This qualitatively indicates thatthe kinetics of the reaction of both MnHCMn and CuHCF with Na⁺ do notchange in the presence of MeCN, up to the case of a 95% MeCN primarysolvent. This example demonstrates that a minimum amount of water mustbe present in the cosolvent electrolyte to allow reversible electrodecycling that yields useful discharge capacity.

Example 6

A MnHCMn electrode was disposed in a half cell in the configurationdescribed above and operated by cyclic voltammetry. In this example, thehalf cell contained an electrolyte of either pure water with no organiccosolvents and 1 M NaClO₄, or of 95% solvent volume basis MeCN, with 5%solvent volume basis water and 1 M or 1.4 M NaClO₄ (FIG. 4). Thebackground current observed at −0.9 V vs. S.H.E. was approximately 1 mAin the aqueous electrolyte containing no organic cosolvents. In the 95%volume basis MeCN electrolytes, the background current at −0.9 V vs.S.H.E. was less than 0.1 mA. Background current during a cyclicvoltammetry scan indicates a side reaction such as the decomposition ofwater that harms coulombic efficiency. This example demonstrates thatthe addition of a majority organic cosolvent results in an improvementin the coulombic efficiency of the MnHCMn anode.

Example 7

A MnHCMn electrode was disposed in a half cell in the configurationdescribed above and operated by galvanostatic cycling at a 1C ratebetween −0.9 V and −0.6 V vs. S.H.E. In aqueous 1 M NaClO₄ containing noorganic cosolvents, the MnHCMn electrode lost 25% of its initialspecific discharge capacity after 15 cycles (FIG. 10). However, in acosolvent electrolyte of 1 M NaClO₄ containing 90% solvent volume MeCNand 10% solvent volume water as a minority cosolvent, less than 5%capacity loss was observed after 15 cycles. This demonstrates that theuse of an organic cosolvent as the majority cosolvent solvent and wateras a minority cosolvent significantly increases the cycle lifetime ofthe MnHCMn anode.

Example 8

A MnHCMn electrode was disposed in a half cell in the configurationdescribed above and operated by galvanostatic cycling at a 1C ratebetween −0.9 V and −0.5 V vs. S.H.E. In aqueous 1 M NaClO₄ containing noorganic cosolvents, the MnHCMn electrode had an initial dischargecapacity of about 40 mAh/g. In 1 M NaClO4 containing 90% solvent volumeMeCN and 10% solvent volume water as a minority cosolvent, a specificdischarge capacity of about 55 mAh/g was achieved. This demonstratesthat the use of the organic primary cosolvent prevents the decompositionof MnHCMn that can result in significant, immediate capacity loss.

Example 9

A MnHCMn electrode was disposed in a half cell in the configurationdescribed above and operated by galvanostatic cycling at a 1C ratebetween −0.95 V and −0.5 V vs. S.H.E. In aqueous 1 M NaClO₄ containingno organic cosolvents, the MnHCMn electrode had coulombic efficiency ofless than 99% (FIG. 12). In 1.4 M NaClO₄ containing 95% solvent volumeMeCN and 5% solvent volume water as a minority cosolvent, a coulombicefficiency of over 99.5% was achieved in three identical cells.

Example 10

A CuHCF electrode was disposed in a half cell in the configurationdescribed above and operated by galvanostatic cycling at a 1C rate.CuHCF loses 4% of its initial capacity after 50 cycles at a 1C rate inaqueous 1 M NaClO₄ (FIG. 13). In contrast, CuHCF is completely stableand shows zero capacity loss after 300 cycles when operated in anelectrolyte of 1 M NaClO₄ containing 90% solvent volume MeCN as theprimary cosolvent and 10% solvent volume water.

Example 11

In this example, MnHCMn and CuHCF electrodes were disposed as anode andcathode, respectively, in a full cell also containing a referenceelectrode as described above. The electrolyte was 1 M NaClO₄ in 90%solvent volume MeCN and 10% solvent volume water. These full cells wereoperated such that the anode was controlled by the reference electrodeas the working electrode. The cathode was oversized such that thecapacity of the anode limited the capacity of the full cell. The MnHCMnanode was galvanostatically cycled at 1C as the working electrodebetween −0.9 V and −0.5 V vs. SHE. Highly reversible cycling of the fullcell is achieved in this primarily organic cosolvent electrolyte (FIG.14). Negligible capacity loss of either the CuHCF cathode or the MnHCMnanode was observed for 30 cycles, as shown by the consistent duration ofeach cycle shown in FIG. 13. This full cell operates at an averagevoltage of 1.7 V, nearly double that of the 0.9 V cell achievable if theupper reaction of MnHCMn is used (FIG. 1, FIG. 7, and FIG. 15). As theelectrode materials in these two cells are identical, and only theirmode of operation is changed, the higher voltage cell offers nearlytwice the energy at the same materials cost. On a basis of the massesand densities of two TMCCC electrodes, a 1.7 V cell will have a specificenergy of 50 Wh/kg and an energy density of 90 Wh/L.

FIG. 16 illustrates a representative secondary electrochemical cell 1600schematic having one or more TMCCC electrodes disposed in contact with acosolvent electrolyte as described herein. Cell 1600 includes a negativeelectrode 1605, a positive electrode 1610 and an electrolyte 1615electrically communicated to the electrodes. One or both of negativeelectrode 1605 and positive electrode 1610 include TMCCC as anelectrochemically active material. A negative current collector 1620including an electrically conductive material conducts electrons betweennegative electrode 1605 and a first cell terminal (not shown). Apositive current collector 1625 including an electrically conductivematerial conducts electrons between positive electrode 1610 and a secondcell terminal (not shown). These current collectors permit cell 1600 toprovide electrical current to an external circuit or to receiveelectrical current/energy from an external circuit during recharging. Inan actual implementation, all components of cell 1600 are appropriatelyenclosed, such as within a protective housing with current collectorsexternally accessible. There are many different options for the formatand arrangement of the components across a wide range of actualimplementations, including aggregation of multiple cells into a batteryamong other uses and applications.

Electrolyte 1615, depending upon implementation, includes a set ofconditions that affect production of hydrogen and oxygen gas responsiveto the operating voltages of the electrodes. In general, at a firstelectrode voltage V1 relative to a reference electrode, initiation ofmore than an incidental quantity of hydrogen gas will begin to beproduced at a particular rate R1 that is consequential for theparticular application. Pure water, under comparable conditions, beginsthe production of hydrogen gas at rate R1 using a second electrodevoltage V2 that is greater than V1 (as shown in FIG. 1, this voltage isless negative). Cell 1600 may be operated at an electrode voltage lessthan V2 but greater than V1 to achieve a greater cell voltage betweenthe electrodes while producing hydrogen gas at second rate R2 less thanR1.

Additives

Electrolytes may contain not only solvents and salts, but additives aswell. These additives may be included for many reasons, including butnot limited to: to improve the calendar or cycle life of one or both ofthe electrodes, to change the coulombic efficiency of one or both of theelectrodes, to change the rate of electrochemical oxidation or reductionof one or both of the electrodes, to prevent chemical or electrochemicaldecomposition of one or more other electrolyte components, to preventchemical dissolution or other chemical reactions of one or both of theelectrodes with the electrolyte or its components, to change the ionicconductivity, viscosity, or transference numbers of the electrolyte, tochange the surface tension of the electrolyte, to change the wetting ofat least one of the separator and electrodes, to change the flammabilityor volatility of the electrolyte, and to decrease the corrosion ordegradation of at least one cell component including the separator,electrodes, current collectors, tabs, terminals, and packaging.

Electrolyte additives may be effective in concentrations as low asnanomolar, or at somewhat higher concentrations such as micromolar, ormillimolar, up to concentrations of about 100 mM. Any particularelectrolyte additive might be most effective at a particularconcentration. Multiple additives might be added to the same electrolytein different concentrations. The most effective concentrations ofadditives and combinations of additives might vary with the salts andcosolvents also present in the electrolyte.

Some electrolyte additives may be chemically active and undergo areaction with another chemical species in the cell, such as with anelectrolyte solvent, salt, or another additive, or such as an electrodecomponent such as an electrochemically active electrode material, abinder, a conductive additive, another electrode additive, or a currentcollector.

Electrolyte additives may be electrochemically active and undergoelectrochemical reactions in the cell. These electrochemical reactionsmay occur on, in, or near one or both electrodes. These reactions may bereversible or irreversible. They may result in the production of a newspecies that is soluble in the electrolyte, or in the production of aninsoluble product on an electrode surface or elsewhere in the cell, orin the production of a gas. Additives may undergo an electrochemicalreaction in which one or more other species of the electrolyte or atleast one electrode are also reactants. These electrochemical reactionsmay occur within the normal operating electrochemical potential rangesof at least one of the electrodes, or they may occur outside of itduring overdischarge or overcharge of the cell. They may also occur whenthe electrolyte is initially added to the cell and before the cell ischarged or discharged for the first time. Finally, they may occur if thetemperature of the cell exceeds its normal range, or if foreignsubstances such as air enter the cell.

Chemical or electrochemical reactions of an electrolyte additive mayalter at least one of the composition, morphology, crystal structure,chemical or electrochemical activity, or other properties of at leastone of the surface or bulk of one or more of the electrodes, separator,current collectors, or other cell components.

The additive or the chemical or electrochemical reaction products of theadditive may change one or more of the chemical or physical propertiesof at least one other cell component such as the solubility, reactivitytowards other species in the cell, thermal stability, thermalconductivity, ionic or electronic conductivity, transference numbers ofthe electrolyte salt species, viscosity, or other properties.

The chemical composition of an electrolyte additive is limited only byits ability to be dissolved, suspended, dispersed, or otherwisedistributed in at least one region of the electrolyte.

An electrolyte additive may include an inorganic component such as ametal cation, including but not limited to alkali metal cations(lithium, sodium, potassium, rubidium, etc.), alkaline earth cations(beryllium, magnesium, calcium, etc.), aluminum, transition metalcations (scandium, titanium, chromium, vanadium, iron, manganese,nickel, cobalt, copper, zinc, zirconium, heavy metal cations (cerium,lead, bismuth, etc.), or intermetallics (gallium, tin, antimony, etc.).These metal cations may have a valence state of one or more of 1+, 2+,3+, 4+, or 5+.

An electrolyte additive may include an inorganic component such as ananion, including but not limited to halogens (fluorine, chlorine,bromine, iodine, etc.), polyatomic anions containing oxygen includingbut not limited to sulfate, nitrate, perchlorate, carbonate, phosphate,and borate, polyatomic anions containing fluorine including but notlimited to tetrafluoroborate, hexafluorophosphate, hexafluoroarsenate,or other inorganic anions. These anions may have a valence state of oneor more of 1−, 2−, 3−, or a more negative valence state.

An electrolyte additive may include a cation containing nitrogen,including but not limited to ammonium, mono substituted ammoniums,disubstituted ammoniums, trisubstituted ammoniums, or tetrasubstitutedammonium. The substituted groups may include linear, branched, or ringedhydrocarbon groups, or groups containing alcohols, ketones,carboxylates, esters, nitriles, or other functional groups. For example,an electrolyte additive might include ethyl ammonium, or tert-butylammonium, or ethyl phenyl ammonium, or tetrabutyl ammonium.

An electrolyte additive may include a polyatomic anion such as acarboxylate (formate, acetate, oxalate, etc.), an alkoxide (ethoxide,isopropoxide, butoxide, etc.), a thiol (decanethiol, etc.), an amine(ethylenediamine, ethylenediaminetetraacetate)

An electrolyte additive may include a neutral organic species such as aquinone (benzoquinone, hydroquinone, etc.), a sulfone (dimethyl sulfone,ethyl methyl sulfone, etc.), a carbonate (propyl carbonate, pentylcarbonate, vinyl carbonate, etc.), ethers including crown ethers,ethylene glycol, polyethyleneglycol, polypropyleneglycol

An electrolyte additive may include an organometallic species such as anorganometallic anion including but not limited to hexacyanoferrate,pentacyanonitrosylferrate, hexacyanocobaltate, hexacyanochromate, or aneutral organometallic species including but not limited to ferrocene.

An electrolyte additive may include a surfactant such as a cationicsurfactant including but not limited to octenidine dihydrochloride,cetrimonium bromide, cetylpyridinium chloride, benzalkonium chloride,benzethonium chloride, distearyl dimethyl ammonium chloride,dioctadecyldimethylammonium bromide, an anionic surfactant including butnot limited ammonium lauryl sulfate, sodium dodecyl sulfate, sodiummyreth sulfate, dioctyl sulfosuccinate, perfluorooctane sulfonate,perfluorobutanesulfonic acid, sodium dodecylbenzenesulfonate, or azwitterionic surfactant including but not limited to sultaines(3-[(3-Cholamidopropyl)dimethylammonio]-1-propanesulfonate) andcocamidopropyl hydroxysultaine, betaines such as cocamidopropylbetainesand phosphatidylethanolamine, phosphatidylchloride and sphingomyelins

An electrolyte additive may include a reducing agent such as aninorganic reducing agent including but not limited to sodiumthiosulfate, sodium dithionite, sodium hydrosulfine, sodium borohydride,sodium cyanoborohydride, sodium triacetoxyborohydride, potassiumborohydride, nickel borohydride, potassium tetrahydroborate, chromium(low valent), indium (low valent), titanium (low valent), iron,phosphorous acid, hydrogen, hydrazine and strontium or an organicreducing agent including but not limited to formic acid, acetic acid,oxalic acid, malic acid, citric acid, 3-mercaptopropionic acid,triphenylphosphite, triphenylphosphine, trimethyl phosphine,triethylphosphine, tributylstannane, tetramethyldisiloxane, sodiumhydroxymethanesulfinate, polymethylhydrosiloxane and formaldehayde.

An electrolyte additive may also include an oxidizing agent such as aninorganic oxidizing agent including but not limited to iron(III),oxygen, oxone, ozone, osmium tetroxide, manganese (IV) oxide, iodine,hydrogen peroxide, chromium trioxide, chlorine, bleach, ammoniumperoxydisulfate, ammonium cerium (IV) nitrate, sodium ferricyanide,potassium permanganate, potassium peroxydisulfate, selium oxide, sodiumbromate, sodium nitrite, sulfur, or an organic oxidizing agent includingbut not limited to quinone, benzoquinone, benzaldehyde, benzyl peroxide,N-Bromosuccinimide, ter-butyl hydroperoxide, tert-butyl nitrite,tert-butyl hypochlorite, dimethylsulfoxide, peracetic acid, pyridineN-oxide. FIG. 17-FIG. 30 illustrate seven pairs of charts correspondingto Example A3-Example A9, each pair of charts including an electrodespotential chart and a full cell voltage chart.

Example A1 (Control with No Additives)

An electrochemical cell was prepared containing two electrodes and anelectrolyte. The first electrode contained a first TMCCC material havinga high electrochemical potential, and the second electrode contained asecond TMCCC material having a low electrochemical potential. For alladditive examples (A1-A9), the first TMCCC material found in the firstelectrode had an approximate composition of Na1.7Fe0.3Mn0.7[Fe(CN)6],and the second TMCCC material found in the second electrode had anapproximate composition of Na2Mn2(CN)6. The electrolyte contained 1 Msodium perchlorate in a solution of 75% sulfolane, 20% acetonitrile, and5% water. The second electrode contained a larger quantity ofelectrochemically active material than did the first electrode. Byapplying constant positive or negative currents to the first electrode,the cell was repeatedly charged to a high voltage, and then dischargedto a low voltage. Because the second electrode contained an excess ofTMCCC material, it was not fully charged by the time the first electrodereached its full charge state. Then, during discharge of the cell, thesecond electrode was over-discharged because it had never reached itsfull charge state during the preceding charging of the cell.

Example A2 (Control with No Additives)

An electrochemical cell was prepared containing two electrodes and anelectrolyte. The first electrode contained a first TMCCC material havinga high electrochemical potential, and the second electrode contained asecond TMCCC material having a low electrochemical potential. Theelectrolyte contained 1 M sodium perchlorate in a solution of 75%sulfolane, 20% acetonitrile, and 5% water. The second electrodecontained a smaller quantity of electrochemically active material thandid the first electrode. By applying constant positive or negativecurrents to the first electrode, the cell was repeatedly charged to ahigh voltage, and then discharged to a low voltage. Because the firstelectrode contained an excess of TMCCC material, it was not fullycharged by the time the second electrode reached its full charge state.

Example A3 (Cu(NO₃)₂ Additive)

An electrochemical cell was prepared containing two electrodes and anelectrolyte. The first electrode contained a first TMCCC material havinga high electrochemical potential, and the second electrode contained asecond TMCCC material having a low electrochemical potential. Theelectrolyte contained 1 M sodium perchlorate and a Cu(NO₃)₂ additive ina 4:1 molar ratio with respect to the second TMCCC material, in asolution of 75% sulfolane, 20% acetonitrile, and 5% water. The secondelectrode contained a smaller quantity of electrochemically activematerial than did the first electrode. By applying constant positive ornegative currents to the first electrode, the cell was repeatedlycharged to a high voltage, and then discharged to a low voltage.Reductive electroplating of the dissolved Cu2+ electrolyte additive toform metallic copper on the surface of the second electrode increasedthe electrochemical charge capacity of that electrode, which allowed thefirst electrode to be charged to a higher potential than as seen inExample A2. As a result, the voltage of the cell was increased, therebyincreasing the energy of the cell. During subsequent charge-dischargecycling, the copper on the surface of the second electrode catalyzed thedecomposition of water in the electrolyte, further increasing theeffective charge capacity of the second electrode. This allowed thefirst electrode to be charged to higher voltages during successivecharge-discharge cycles, resulting in further increases to the cellenergy as charge-discharge cycling continued. FIG. 17-FIG. 18 illustratea first pair of charts for Example A3 comparing a control (no additive)to a Cu(NO₃)₂ additive, FIG. 17 illustrates an electrode potentialschart for Example A3, and FIG. 18 illustrates a cell voltage chart forExample A3.

Example A4 (Benzoquinone Additive)

An electrochemical cell was prepared containing two electrodes and anelectrolyte. The first electrode contained a first TMCCC material havinga high electrochemical potential, and the second electrode contained asecond TMCCC material having a low electrochemical potential. Theelectrolyte contained 1 M sodium perchlorate and a benzoquinone additivein a 4:1 molar ratio with respect to the second TMCCC material, in asolution of 75% sulfolane, 20% acetonitrile, and 5% water. The secondelectrode contained a smaller quantity of electrochemically activematerial than did the first electrode. By applying constant positive ornegative currents to the first electrode, the cell was repeatedlycharged to a high voltage, and then discharged to a low voltage.Reduction of the dissolved benzoquinone electrolyte additive by thesecond electrode to form hydroquinone increased the electrochemicalcharge capacity of that electrode, which allowed the first electrode tobe charged to a higher potential than as seen in Example A2. As aresult, the voltage of the cell was increased, thereby increasing theenergy of the cell. During subsequent charge-discharge cycling, thesecond electrode continued to reduce the remaining benzoquinone, furtherincreasing the effective charge capacity of the second electrode. Thisallowed the first electrode to be charged to higher voltages duringsuccessive charge-discharge cycles, resulting in further increases tothe cell energy as charge-discharge cycling continued. FIG. 19-FIG. 20illustrate a second pair of charts for Example A4 comparing a control(no additive) to a Benzoquinone additive, FIG. 19 illustrates anelectrode potentials chart for Example A4, and FIG. 20 illustrates acell voltage chart for Example A4.

Example A5 (Hydroquinone Additive)

An electrochemical cell was prepared containing two electrodes and anelectrolyte. The first electrode contained a first TMCCC material havinga high electrochemical potential, and the second electrode contained asecond TMCCC material having a low electrochemical potential. Theelectrolyte contained 1 M sodium perchlorate and a hydroquinone additivein a 4:1 molar ratio with respect to the first TMCCC material, in asolution of 75% sulfolane, 20% acetonitrile, and 5% water. The secondelectrode contained a larger quantity of electrochemically activematerial than did the first electrode. By applying constant positive ornegative currents to the first electrode, the cell was repeatedlycharged to a high voltage, and then discharged to a low voltage.Oxidation of the dissolved hydroquinone electrolyte additive by thefirst electrode to form benzoquinone increased the electrochemicalcharge capacity of that electrode, which allowed the second electrode tobe charged to a lower potential than as seen in Example A1. As a result,the voltage of the cell was increased, thereby increasing the energy ofthe cell. During subsequent charge-discharge cycling, the firstelectrode continued to oxidize the remaining hydroquinone, furtherincreasing the effective charge capacity of the first electrode. Thisallowed the second electrode to be charged to lower voltages duringsuccessive charge-discharge cycles, resulting in further increases tothe cell energy as charge-discharge cycling continued. FIG. 21-FIG. 22illustrate a third pair of charts for Example A5 comparing a control (noadditive) to a Hydroquinone additive, FIG. 21 illustrates an electrodepotentials chart for Example A5, and FIG. 22 illustrates a cell voltagechart for Example A5.

Example A6 (Ferrocene Additive)

An electrochemical cell was prepared containing two electrodes and anelectrolyte. The first electrode contained a first TMCCC material havinga high electrochemical potential, and the second electrode contained asecond TMCCC material having a low electrochemical potential. Theelectrolyte contained 1 M sodium perchlorate and a ferrocene additive ina 4:1 molar ratio with respect to the first TMCCC material, in asolution of 75% sulfolane, 20% acetonitrile, and 5% water. The secondelectrode contained a larger quantity of electrochemically activematerial than did the first electrode. By applying constant positive ornegative currents to the first electrode, the cell was repeatedlycharged to a high voltage, and then discharged to a low voltage.Oxidation of the dissolved ferrocene electrolyte additive by the firstelectrode to form ferrocenium increased the electrochemical chargecapacity of that electrode, which allowed the second electrode to becharged to a lower potential than as seen in Example A1. As a result,the voltage of the cell was increased, thereby increasing the energy ofthe cell. During subsequent charge-discharge cycling, the firstelectrode continued to oxidize the remaining ferrocene, furtherincreasing the effective charge capacity of the first electrode. Thisallowed the second electrode to be charged to lower voltages duringsuccessive charge-discharge cycles, resulting in further increases tothe cell energy as charge-discharge cycling continued. FIG. 23-FIG. 24illustrate a fourth pair of charts for Example A6 comparing a control(no additive) to a Ferrocene additive, FIG. 23 illustrates an electrodepotentials chart for Example A6, and FIG. 24 illustrates a cell voltagechart for Example A6.

Example A7 (Cu(NO₃)₂ Additive)

An electrochemical cell was prepared containing two electrodes and anelectrolyte. The first electrode contained a first TMCCC material havinga high electrochemical potential, and the second electrode contained asecond TMCCC material having a low electrochemical potential. Theelectrolyte contained 1 M sodium perchlorate and a Cu(NO₃)₂ additive ina 1:1 molar ratio with respect to the second TMCCC material, in asolution of 75% sulfolane, 20% acetonitrile, and 5% water. The secondelectrode contained a smaller quantity of electrochemically activematerial than did the first electrode. By applying constant positive ornegative currents to the first electrode, the cell was repeatedlycharged to a high voltage, and then discharged to a low voltage.Reductive electroplating of the dissolved Cu2+ electrolyte additive toform metallic copper on the surface of the second electrode increasedthe electrochemical charge capacity of that electrode, which allowed thefirst electrode to be charged to a higher potential than as seen inExample A2. As a result, the voltage of the cell was increased, therebyincreasing the energy of the cell. During subsequent charge-dischargecycling, the copper on the surface of the second electrode catalyzed thedecomposition of water in the electrolyte, further increasing theeffective charge capacity of the second electrode. This allowed thefirst electrode to be charged to higher voltages during successivecharge-discharge cycles, resulting in further increases to the cellenergy as charge-discharge cycling continued. FIG. 25-FIG. 26 illustratea fifth pair of charts for Example A7 comparing a control (no additive)to a Cu(NO₃)₂ additive, FIG. 25 illustrates an electrode potentialschart for Example A7, and FIG. 26 illustrates a cell voltage chart forExample A7.

Example A8 (Oxalic Acid Additive)

An electrochemical cell was prepared containing two electrodes and anelectrolyte. The first electrode contained a first TMCCC material havinga high electrochemical potential, and the second electrode contained asecond TMCCC material having a low electrochemical potential. Theelectrolyte contained 1 M sodium perchlorate and an oxalic acid additivein a 1:1 molar ratio with respect to the second TMCCC material, in asolution of 75% sulfolane, 20% acetonitrile, and 5% water. The secondelectrode contained a smaller quantity of electrochemically activematerial than did the first electrode. By applying constant positive ornegative currents to the first electrode, the cell was repeatedlycharged to a high voltage, and then discharged to a low voltage. Anelectrochemical reaction of the dissolved oxalic acid electrolyteadditive with the second electrode increased the electrochemical chargecapacity of that electrode, which allowed the first electrode to becharged to a higher potential than as seen in Example A2. As a result,the voltage of the cell was increased, thereby increasing the energy ofthe cell. During subsequent charge-discharge cycling, the copper on thesurface of the second electrode catalyzed the decomposition of water inthe electrolyte, further increasing the effective charge capacity of thesecond electrode. This allowed the first electrode to be charged tohigher voltages during successive charge-discharge cycles, resulting infurther increases to the cell energy as charge-discharge cyclingcontinued. FIG. 27-FIG. 28 illustrate a sixth pair of charts for ExampleA8 comparing a control (no additive) to an Oxalic acid additive, FIG. 27illustrates an electrode potentials chart for Example A8, and FIG. 28illustrates a cell voltage chart for Example A8.

Example A9 (Pyrrole Additive)

An electrochemical cell was prepared containing two electrodes and anelectrolyte. The first electrode contained a first TMCCC material havinga high electrochemical potential, and the second electrode contained asecond TMCCC material having a low electrochemical potential. Theelectrolyte contained 1 M sodium perchlorate and a pyrrole additive in a0.4:1 molar ratio with respect to the first TMCCC material, in asolution of 75% sulfolane, 20% acetonitrile, and 5% water. The secondelectrode contained a larger quantity of electrochemically activematerial than did the first electrode. By applying constant positive ornegative currents to the first electrode, the cell was repeatedlycharged to a high voltage, and then discharged to a low voltage.Oxidative polymerization of the pyrrole electrolyte additive at thesurface of the first electrode increased the electrochemical chargecapacity of that electrode, which allowed the first electrode to becharged to a higher potential than as seen in Example A2. As a result,the voltage of the cell was increased, thereby increasing the energy ofthe cell. During subsequent charge-discharge cycling, the copper on thesurface of the second electrode catalyzed the decomposition of water inthe electrolyte, further increasing the effective charge capacity of thesecond electrode. This allowed the first electrode to be charged tohigher voltages during successive charge-discharge cycles, resulting infurther increases to the cell energy as charge-discharge cyclingcontinued. FIG. 29-FIG. 30 illustrate a seventh pair of charts forExample A9 comparing a control (no additive) to a Pyrrole additive, FIG.29 illustrates an electrode potentials chart for Example A9, and FIG. 30illustrates a cell voltage chart for Example A9.

As demonstrated in Examples A1-A9, more particularly in Examples A3-A9,one or more side-reactions exist with an additive-containing electrolyteand one or more electrodes coupled to the electrolyte. Theseside-reactions may decrease coulombic efficiency of charging anelectrode (e.g., degrade it) and do not appear in these examples tocompletely suspend charging for a duration of the side-reaction. It maybe the case that some embodiments include an ability to suspend chargingfor some period, such as a case where the side-reaction occurs with muchgreater efficiency than the actual charging of the electrode. Theefficiency of the side-reaction may be increased or decreased as desiredby increasing or decreasing a concentration of the additive in theelectrolyte.

As noted herein, these side-reactions may be reversible or irreversible.Further, these additives may part of an electrode or electrode assembly.To be reversible, it may be necessary for the additive to dissolve ordisassociate into the electrolyte, diffuse across the cell, and reactwith another electrode. In this fashion, it may be possible in someembodiments to reset to its initial redox state after consumption duringthe side-reaction.

These side-reactions may include chemical and electrochemical reactions.The additive could undergo a chemical reaction with one of theelectrodes. For example, the additive could bond to the surface of theelectrode, and then act as a catalyst for reactions with the electrolytethat decrease the charging efficiency.

The additive could also, or in lieu of, undergo an electrochemicalreaction with one of the electrodes. For example, an embodiment may addCu²⁺ cations to the electrolyte, then the additive may be reducedelectrochemically on the surface of an electrode to form copper metal:Cu²⁺+2e−=Cu.

In the case of an additive that undergoes reversible reactions, it maybe more likely that these would be electrochemical. For example, shouldan embodiment add a ferrocene additive to the electrolyte, it could bestable at the anode, but it may oxidize at the cathode to ferrocinium.That ferrocinium would be stable at the cathode, but it would reduceback to ferrocene at the anode. Then, that newly reduced ferrocene coulddiffuse back to the cathode and be reoxidized to ferrocinium. Thiscyclic oxidation and reduction of ferrocene at the two electrodes maythen continue for the entire duration of operation of the cell.

In some secondary storage systems, there may be a desire to improvecoulombic efficiencies by suppressing undesired side-reactions. Incontrast, the embodiments described herein intentionally add furtherinefficiencies in a manner that is beneficial to the operation of thesystem. For example, one discovered benefit is that in some embodimentsit may be easier to balance inefficiencies of two electrodes when one ormore of the inefficiencies are larger.

Regarding a duration of these side-reactions, as long as there is aquantity of unconsumed additive present during charging, the chargingefficiency is degraded. In the cases of a reversible side-reactionhaving sufficient additive to last an entire period of charging, theneach charging and discharging cycle may replenish the additive and allowconsistent repeatable charging degradation for the entire operationallife of the cell. Even in an embodiment that includes a very largequantity of an additive that is consumed by an irreversible reaction, itmight take the entire first charge, or even a number of successivecharge/discharge cycles to consume the additive fully. The additive maybe periodically replenished based upon the quantity, rate ofconsumption, and operation of a device. In some designs forelectrochemical devices, there may be a port through which moreelectrolyte is added to the cell. An embodiment of the present inventioncould implement or take advantage of such a feature by “topping off” thecell with more additive by adding an additive, an additive precursor,and/or an electrolyte having such additive or additive precursor intothe device through the port.

While the mechanism for degrading charging efficiency described hereinhas focused primarily on chemical and electrochemical side-reactions,some embodiments may employ something else in addition, or as analternative. That something else may include, for example, a case inwhich one electrode might burn off charge by reacting with theelectrolyte itself at very high temperature. Or, it might undergo aspontaneous phase change at very high temperature. Thus, heating mightbe a substitute for an electrolyte additive. Other substitutes may alsoexist in some embodiments to replace or supplement an additive.

FIG. 31 illustrates a set of charts for charging and discharging under aset of different cases. In FIG. 31, there is a representation ofPotential (V) vs. Arbitrary Reference Electrode as a function of time(T) and a representation of Full Cell Voltage (V) as a function time (T)for four different cases: case 1, case 2, case 3, and case 4. Each caseincludes an electrode charge phase (for a cathode and an anode) followedby an electrode discharge phase. The cathode charge phase includes aninitial state-of-charge (A) charging to a maximum state-of-charge (B)with the cathode discharge phase including the SOC B discharging to afinal state-of-charge (C). The anode charge phase includes an initialstate-of-charge (D) charging to a maximum negative state-of-charge (E)with the anode discharge phase including the SOC E discharging to afinal state-of-charge (F). The full cell voltage illustrates a full cellvoltage at a beginning of the charge phase (G), completion of thecharging and beginning of discharging (H), and a completion of theelectrode discharge phase (I).

Case 1 represents an ideal case where the cell is able to fully chargeand discharge. In case 1, an electrolyte (or other mechanism) does notlimit cell charging. Both electrodes can fully charge and dischargebetween +1.0 V and +1.5 V and −1.0 and −1.0 V and −1.5 V, resulting in afull cell voltage that ranges between 2.0 V and 3.0 V with 100% capacityutilization. Case 2 represents a non-ideal case with both electrodesstarting at the same initial state-of-charge and experiencing equalcharge efficiencies. For example, an electrolyte stability range is 2.8V (not 3.0 V as in case 1). When both electrodes start as the same SOC,then they reach maximum SOCs of 80% and the cell experiences 80%capacity utilization. 80% capacity utilization is optimum for thiselectrolyte with this set of conditions. Case 3 represents a non-idealcase with the electrodes starting at different SOCs and experiencingequal charge efficiencies. The electrolyte stability range is 2.8 V andthe cathode starts at a higher SOC (40%) than the anode (0%). With thisset of conditions, the anode reaches a maximum SOC of 60% when the cellis fully charged which reduces the capacity utilization to 60%, which isless than optimum for this cell (for example, compare to case 2). Case 4represents a modification to case 3 by inclusion of an additive to theelectrode that decreases a charging efficiency of the cathode by 50%.The boundary and initial conditions of case 4 are the same as case 3except for the selective modification to the cathode charging efficiency(e.g., such as by use of a side-reaction as described herein). In thiscase 4, the cathode charges from 40% to 80% SOC while the anode chargesfrom 0% to 80%, producing an optimum capacity utilization for the cellby the use of the additive. Table I below summarizes the specific casestates-of-charge (SOC) and cell utilization:

TABLE I SOC/Utilization by Case No. Capacity CASE A (%) B (%) C (%) D(%) E (%) F (%) G (%) H (%) I (%) Util (%) 1 0 100 0 0 100 100 0 100 0100 2 0 80 0 0 80 0 0 80 0 80 3 40 100 40 0 60 0 0 60 0 60 4 40 80 0 080 0 0 80 0 80

In case 1, the cell may reach a full charge at 3.0 volts, with thecathode reaching a potential of 1.5 volts at full charge and the anodereaching a potential of −1.5 volts at full charge, with respect to thereference electrode. During a discharge of this cell to 2.0 volts, thecathode may discharge fully to a potential of 1.0 volts and the anodemay discharge fully to a potential of −1.0 volts with respect to thereference electrode.

In case 2 with a cell reaching only 2.8 volts, when both electrodes havethe same initial state of charge and equal charge efficiencies, then thecathode may charge to 80% state of charge at 1.4 volts with respect tothe reference electrode and the anode to 80% state of charge at −1.4volts with respect to the reference electrode. During a subsequentdischarge, the cell may discharge 80% of its theoretical capacity beforeeither of the two electrodes reaches a state of charge of zero.

However, there instead may be an unbalanced charging that limits thedischarge capacity of the cell (case 3). In case 3, the cathode may havean initial state of charge of 40% at 1.2 volts with respect to thereference electrode, while the anode may have an initial state of chargeof zero at −1.0 volts with respect to the reference electrode. In thiscase, the full cell may reach its maximum voltage of 2.8 volts with thecathode fully charging to 100% state of charge 1.5 volts with respect tothe reference electrode, and the anode partially charging to 60% at −1.3volts with respect to the reference electrode (a relativestate-of-charge imbalance of 40% between the electrodes). During asubsequent discharge, the cell may discharge only 60% of its theoreticalcapacity before the anode reaches a state of charge of zero. In case 3,the discharge capacity of the cell is not optimized for the boundarycondition of an electrolyte having a 2.8 volt stability window becausethe anode has been charged to a lower state of charge than the cathodewhen the cell voltage reaches 2.8 volts, so there is less dischargecapacity available.

The maximum discharge capacity is achieved only when the states ofcharge of the cathode and anode are equal at the maximum cell voltage.This may be achieved by use of a limited side-reaction in which thecoulombic efficiency of at least one electrode is degraded. For example,consider case 4 in which the cathode has an initial state of charge of40% at 1.2 volts with respect to the reference electrode, the anode hasan initial state of charge of zero, at −1.0 volts with respect to thereference electrode, and in which an additive to the electrolytedegrades the coulombic efficiency of the charging of the cathode to 50%of the coulombic efficiency of the charging of the anode. In this case4, during charging of the cell the state of charge of the cathode mayincrease from 40% to 80% while the state of charge of the anode mayincrease from zero to 80%, at which point the cell reaches a maximumvoltage of 2.8 volts. During a subsequent discharge, the cell maydischarge 80% of its theoretical capacity before either electrodereaches a state of charge of zero. In this case 4, the addition of anelectrolyte additive to degrade the coulombic efficiency of charging ofthe cathode allowed the electrodes to reach a balanced state of chargewhen the cell reached its maximum voltage, which resulted inoptimization of the discharge capacity of the cell.

Some embodiments relate to a class of electrode materials having stiffopen framework structures into which hydrated cations can be reversiblyand rapidly intercalated from aqueous (e.g., water-based) electrolytesor other types of electrolytes. In particular, open framework structureswith the Prussian Blue-type crystal structure afford advantagesincluding greater durability and faster kinetics when compared to otherintercalation and displacement electrode materials. A general formulafor a TMCCC/PBA class of materials is given by:

A_(x)P_(y)[R(CN)_(6-j)L_(j)]_(z) .nH₂O, where:

A is a monovalent cation such as Na⁺, K⁺, Li⁺, or NH₄ ⁺, or a divalentcation such as Mg²⁺ or Ca²⁺;P is a transition metal cation such as Ti³⁺, Ti⁴⁺, V²⁺, V³⁺, Cr²⁺, Cr³⁺,Mn⁺, Mn²⁺, Mn³⁺, Fe²⁺, Fe³⁺, Co²⁺, Co³⁺, Ni²⁺, Cu⁺, Cu²⁺, or Zn²⁺, oranother metal cation such as Al³⁺, Sn²⁺, In³⁺, or Pb²⁺;R is a transition metal cation such as V²⁺, V³⁺, Cr²⁺, Cr³⁺, Mn⁺, Mn²⁺,Mn³⁺, Fe²⁺, Fe³⁺, Co²⁺, Co³⁺, Ru²⁺, Ru³⁺, Os²⁺, Os³⁺, Ir²⁺, Ir³⁺, Pt²⁺,or Pt³⁺;L is a ligand that may be substituted in the place of a CN⁻ ligand,including CO (carbonyl), NO (nitrosyl), or Cr;0≤x≤2;0<y≤4;0<z≤1;0≤j≤6; and0≤n≤5;wherein j is typically zero but may have a non-integer value when thePBA material includes a mixture of multiple types of R(CN)_(6-j)L_(j)groups, for example half R(CN)₆ and half R(CN)₅L₁, then the average inthe whole material is a non-integer j.

FIG. 32 illustrates a unit cell of the Prussian Blue crystal structure.The unit cell of copper hexacyanoferrate, a TMCCC. In this material andall other TMCCCs, transition metal cations are linked in a face centeredcubic framework by cyanide bridging ligands. In this case, iron issix-fold carbon coordinated, while copper is six-fold nitrogencoordinated. Each unit cell contains eight smaller cubic subcells, atthe center of which is a large interstitice designated as the “A Site”.The A Sites contain zeolitic water and mobile alkali cations such as Na⁺or K⁺ or NH₄ ⁺. During the electrochemical cycling of a TMCCC, alkalications are inserted or removed from the A Sites.

FIG. 33 illustrates an X-ray diffraction spectrum of CuHCF. The fullyindexed powder X-ray diffraction spectra of copper hexacyanoferrate andPrussian Blue. Copper hexacyanoferrate has the well-known face-centeredcubic open framework structure of Prussian Blue.

FIG. 34 illustrates a micrograph of CuHCF. Scanning electron microscopyof copper hexacyanoferrate shows that the material is composed ofagglomerations of 20-50 nm grains. These agglomerations can be as largeas several microns.

FIG. 35 illustrates X-ray diffraction spectra of MnHCMn. The powderX-ray diffraction spectrum of freshly synthesized, fully reducedmanganese(II) hexacyanomanganate(II), and of the same material afterpartial oxidation. In the latter case, a symmetry-breaking distortion inthe framework structure is eliminated during oxidation, forming the morecommon face-centered cubic phase (as in FIG. 32 and FIG. 33).

FIG. 36 illustrates a micrograph of MnHCMn. Scanning electron microscopyof manganese hexacyanomanganate, as synthesized by a simple, one-stepsynthesis method.

FIG. 37 illustrates baseline/control electrochemical cycling of CuHCF.The fractional capacity retention of copper hexacyanoferrate duringgalvanostatic cycling at a 1C rate between 0.8 and 1.05 V vs. standardhydrogen electrode (SHE) in 1 M KNO₃ (pH=2) with a Ag/AgCl referenceelectrode and an activated charcoal counter electrode. These two cellsrepresent a consistently observed loss of 7%/50 cycles under theseconditions.

FIG. 38 illustrates a UV-visible spectrum of CuHCF in water and 1 M KNO₃pH=2. Ultraviolet-visible absorbance spectroscopy of aqueous solutionsthat had contained 1 mg CuHCF electrode per 1 g of solution for 24hours. The presence of concentrated K+ drastically reduces the solubleferricyanide signal (peak at 420 nm).

FIG. 39 illustrates an ultraviolet-visible absorbance spectrum of CuHCFin water and 10 mM Cu²⁺. The ultraviolet-visible absorbance spectra ofsolutions that had contained 1 mg CuHCF electrode per 1 g of solutionfor 24 hours. The addition of dilute (10 mM) copper nitrate results in anear-total elimination of the absorbance peak due to solubleferricyanide. This demonstrates that P^(m+) electrolyte additives slowor prevent the dissolution of APR(CN)₆ TMCCCs.

FIG. 40 illustrates the cycle life of CuHCF in 1 M KNO₃ pH=2 with andwithout Cu²⁺ added. Galvanostatic cycling of copper hexacyanoferrateagainst a metallic copper anode in 1 M KNO₃/0.1 M Cu(NO₃)₂ (pH=2) at a1C rate results in a steep initial capacity loss, followed bystabilization of the electrode, with zero capacity loss between cycle 20and cycle 25. In contrast, the rate capacity loss observed in a controlcell containing an activated charcoal anode and no Cu(NO₃)₂ in theelectrolyte is constant. After 20 cycles, the rate of the continuingcapacity loss is greater in the control cell than in the cell containingthe Cu(NO₃)₂ electrolyte additive and the Cu metal anode.

FIG. 41 illustrates galvanostatic cycling of CuHCF/Cu²⁺/Cumetal in 2sub-figures. FIG. 41a ): The potential profiles of the copperhexacyanoferrate cathode and the copper anode, and the full cellvoltage, during galvanostatic cycling at a 1C rate in 1 M KNO₃ (pH=2)with 0.1 M Cu(NO₃)₃ added. FIG. 41b ) The same data, plotted as afunction of the specific capacity of the copper hexacyanoferratecathode. Cycling is highly reversible.

FIG. 42 illustrates cyclic voltammetry of CuHCF and PB/BG. Cyclicvoltammetry (scan rate 1 mV/s) of copper hexacyanoferrate and PrussianBlue electrodes in 1 M KNO₃ (pH=2) electrolyte. The reaction potentialof copper hexacyanoferrate is centered at 0.95 V, while the oxidation ofPrussian Blue to Berlin Green is centered at nearly 1.2 V. This meansthat copper hexacyanoferrate can be fully oxidized before appreciableoxidation of the Prussian Blue occurs. In the case of a Prussian Bluecoating on a copper hexacyanoferrate electrode, the electrode can becharged and discharged without oxidizing the coating.

FIG. 43 illustrates capacity retention of PB/CuHCF and CuHCF. Copperhexacyanoferrate electrodes that have been coated by a thin film ofPrussian Blue have improved capacity retention in comparison to uncoatedelectrodes.

FIG. 44 illustrates capacity retention of CuHCF w/K⁺ in PB dep solution.The capacity retention of copper hexacyanoferrate does not improve ifthe electrodes are exposed to a solution containing both Prussian Blueprecursors and a more concentrated potassium salt.

FIG. 45 illustrates potential profiles of CuHCF and Prussian Blue-coatedCuHCF electrodes. The potential profiles of bare and PrussianBlue-coated copper hexacyanoferrate electrodes. The coating does nothave an appreciable effect on the electrochemical behavior of theelectrode.

FIG. 46 illustrates morphologies of bare and Prussian Blue-coated CuHCFelectrodes in two sub-figures: FIG. 46a ) illustrates scanning electronmicroscopy of a freshly deposited slurry electrode of copperhexacyanoferrate (80%), carbon black (10%), and polyvinylidenedifluoride (10%) on a carbon cloth substrate and FIG. 46b ) illustratesthe same sample, after electrochemical reduction, followed by 40 minutesof exposure to a 2 mM aqueous solution of Fe(CN)₃ and K₃Fe(CN)₆. A filmof Prussian Blue has clearly precipitated on the surface of the sample,as the grains at the surface are larger and form a more continuoussurface than seen in FIG. 46a ).

FIG. 47 illustrates cycle life of CuHCF with PB coating on theparticles. Fractional capacity retention of a standard copperhexacyanoferrate electrode, and an electrode containing PrussianBlue-coated copper hexacyanoferrate during galvanostatic cycling at a 1Crate in 1 M KNO₃ (pH=2). The use of a Prussian Blue coating stabilizesthe individual copper hexacyanoferrate particles against dissolution.

FIG. 48 illustrates potential profiles of CuHCF with PB coating on theparticles in two sub-figures: FIG. 48a illustrates the potentialprofiles of electrodes containing untreated copper hexacyanoferrate, andcopper hexacyanoferrate nanoparticles coated with Prussian Blue, duringgalvanostatic cycling at a 1C rate in 1 M KNO₃ (pH=2) and FIG. 48billustrates Galvanostatic cycling of an electrode containingPrussian-Blue coated copper hexacyanoferrate nanoparticles over a widerpotential range. Prussian Blue is electrochemically active at 0.4 V vs.SHE. About 20% of the total capacity of the electrode occurs at lowpotential, indicating a ratio of copper hexacyanoferrate to PrussianBlue of about 4:1. This is in agreement with the 4:1 ratio of copperhexacyanoferrate to Prussian Blue precursors added during the coatingtreatment procedure.

FIG. 49 illustrates the cycle life of CuHCF with PPy coating on theparticles. An electrode containing polypyrrole-coated copperhexacyanoferrate shows a completely stable capacity for at least 50cycles at a 1C rate in 1 M KNO₃ (pH=2). In comparison, a control samplecontaining uncoated copper hexacyanoferrate loses about 7% of itscapacity after the same duration of cycling.

FIG. 50 illustrates potential profiles of CuHCF with PPy coating on theparticles. A large, irreversible charge capacity is observed during thefirst charge of polypyrrole-coated copper hexacyanoferrate. Cyclingafter the first charge is extremely reversible. The reversible reactioncentered at 0.95 V vs. SHE is consistent with the one observed foruncoated copper hexacyanoferrate, showing that the polypyrrole coatingis inactive in this potential range.

A battery (or cell) comprises an anode, a cathode, and an electrolytethat is in contact with both the anode and the cathode. Both the cathodeand the anode contain electrochemically active material that may undergoa change in valence state, accompanied by the acceptance or release ofcations and electrons. For example, during discharge of a battery,electrons are extracted from the anode to an external circuit, whilecations are removed from the anode into the electrolyte. Simultaneously,electrons from the external circuit enter the cathode, as do cationsfrom the electrolyte. The difference in the electrochemical potentialsof the cathode and anode results in a full cell voltage. This voltagedifference allows energy to be extracted from the battery duringdischarge, or stored in the battery during charge.

Prussian Blue is a well-known material phase of iron cyanide hydrate ofthe chemical formula K_(x)FeIII[FeII(CN)₆]_(z).nH₂O (0≤x, 0<z≤1; n≈4).This material has been produced industrially for centuries for use as apigment and dyestuff. It is also a well-known electrochromic material,and has been studied for use as a cathode in electrochromic displays.FIG. 32 illustrates Prussian Blue as having a face-centered cubiccrystal structure. In this structure, cyanide bridging ligands linktransition metal cations in a spacious open framework. The structurecontains large interstitial sites commonly called the “A Sites.” Eachunit cell contains eight A Sites, each of which may contain zeoliticwater, interstitial alkali cations, or both.

For example, copper hexacyanoferrate (CuHCF) is a TMCCC recentlydemonstrated to be a high performance battery electrode. In the openframework structure of CuHCF, iron is six-fold, octahedrally coordinatedto the carbon ends of the cyanide branching ligands, while copper isoctahedrally nitrogen-coordinated as shown in FIG. 32. Depending on themethod of synthesis, the A sites in CuHCF may contain potassium oranother alkali cation such as sodium or lithium, or another type ofcation such as ammonium. More generally, for a TMCCC of the generalchemical formula A_(x)P_(y)[R(CN)₆]_(z).nH₂O, alkali cations A⁺ andwater occupy the interstitial A Sites, transition metal P cations aresix-fold nitrogen coordinated, and transition metal R cations aresix-fold carbon coordinated.

Herein the electrochemical cells used to test electrode propertiescontained a TMCCC working electrode, a counter electrode, an electrolytein contact with both the anode and cathode, and a Ag/AgCl referenceelectrode used to independently measure the potentials of the anode andcathode during charge and discharge of the cell. When the electrode ofinterest was a cathode material, then the working electrode was thecathode, and the counter electrode was the anode. When the electrode ofinterest was an anode material, then the working electrode was theanode, and the counter electrode was the anode. In the case that thecell did not contain both a TMCCC cathode and a TMCCC anode, acapacitive activated charcoal counter electrode was used to complete thecircuit while allowing the study of a single TMCCC electrode.

Several measurement and characterization techniques were used to examinethe materials and electrodes described here. Physical characterizationof TMCCC materials was performed using X-ray diffraction (XRD) andscanning electron microscopy (SEM). Electrochemical characterization ofelectrodes was performed using galvanostatic cycling with potentiallimitation (GCPL). During the GCPL technique a constant current isapplied to the cell until the working electrode reaches a maximum orminimum potential; upon reaching this extreme potential, the sign of thecurrent is reversed.

In the application, sometimes a shorthand reference is made to a“standard” method for materials synthesis. Those references include thisfollowing discussion, sometimes the context includes a modification oradjustment of a portion of this synthesis. CuHCF was synthesized usingexisting techniques, such disclosed in Wessells, C. D., et al. Copperhexacyanoferrate battery electrodes with long cycle life and high power.Nature Comm., 2, 550 (2011). An aqueous solution of Cu(NO₃)₂, and asecond aqueous solution of K₃Fe(CN)₆ were added to water bysimultaneous, dropwise addition while stirring. The final concentrationsof the precursors were 40 mM Cu(NO₃)₂ and 20 mM K₃Fe(CN)₆. A solid,brown precipitate formed immediately. It was filtered or centrifuged,washed, and dried. In a prior study, CuHCF synthesized by this methodwas found to have the composition K_(0.7)Cu[Fe(CN)₆]_(0.7).2.8H₂O. FIG.33 illustrates that CuHCF was found to have the cubic Prussian Blue openframework crystal structure using XRD. The CuHCF was composed ofnanoparticles about 50 nm in size, as verified by SEM as shown in FIG.34.

Manganese hexacyanomanganate (MnHCMn) was synthesized using asingle-step procedure such as disclosed in Her, J.-H., et al. AnomalousNon-Prussian Blue Structures and Magnetic Ordering of K₂MnII[MnII(CN)₆]and Rb₂MnII[MnII(CN)₆ ]. Inorg. Chem., 49, 1524 (2010). A 10 mL aqueoussolution containing 0.5 g KCN was slowly added to a 10 mL aqueoussolution containing 0.5 g of MnCl₂ in a N₂ atmosphere. A dark greenprecipitate slowly formed. This precipitate was centrifuged, washed, anddried with no exposure to air or oxygen. X-ray diffraction of thefreshly synthesized material revealed a monoclinic structure indicativeof a slight distortion to the standard Prussian Blue open frameworkstructure as shown in FIG. 35. After partial oxidation, the cubic phasewas found to form. This result indicates an approximate chemical formulaK₂MnII[MnII(CN)₆].nH₂O. SEM of FIG. 36 illustrates that the MnHCMn wascomposed of 1-5 μm agglomerations of 200-1000 nm particles.

Aqueous electrolytes were prepared from reagent-grade salts such as KNO₃or NaClO₄ and de-ionized water. These alkali salt electrolytes aretypically pH-neutral. For cases in which the electrolytes wereacidified, the pH was lowered using HNO₃.

Electrodes containing the freshly synthesized TMCCCs were prepared usingvarious techniques known in the art. The electrochemically activematerial, carbon black, and polyvinylidene difluoride (PVDF) binder wereground by hand until homogeneous, and then stirred in1-methyl-2-pyrrolidinone (NMP) solvent for several hours. This slurrywas deposited on electronically conductive substrates such as aluminumfoil or carbon cloth using a doctor blade or spatula. These electrodeswere dried in vacuum or a N₂ atmosphere at 60° C.

Activated charcoal counter electrodes were prepared by grinding thecharcoal with PVDF before stirring in NMP for several hours, followed bydeposition and drying on conductive substrates following the sameprocedure as in the case of electrodes containing a TMCCC.

As a control test, CuHCF electrodes (5 mg CuHCF) were cycled at a 1Crate (one hour charge or discharge) by GCPL between 0.8 and 1.05 V withrespect to the standard hydrogen electrode (SHE) in a cell that alsocontained a Ag/AgCl reference electrode, an activated charcoal counterelectrode, and 15 mL of aqueous 1 M KNO₃ (pH=2) electrolyte. FIG. 37illustrates that about 7% capacity loss is observed after 50 cycles.

Electrode Life Extension Method 1: P Electrolyte Additives

The dissolution of a TMCCC occurs by the following general process:APR(CN)₆→A⁺+P^(m+)+R(CN)₆ ^(n−) where A is an alkali cation, P and R aretransition metal cations, and n=−1·(m+1). The dissolution process willcontinue until the saturation limit of the dissolution products isreached. At this chemical equilibrium, the thermodynamic driving forcefor further dissolution is zero.

The thermodynamic driving force for a chemical process occurring atconstant temperature and pressure is the change in the Gibbs Free Energy(ΔG). It is related to the equilibrium constant (Keq) of a reaction bythe following expression: ΔG=−R·T·ln(Keq), where R is the ideal gasconstant and T is the absolute temperature. The equilibrium constant forthe dissolution of a TMCCC is the product of the chemical activities ofthe dissolution products, divided by the chemical activity of solidTMCCC: Keq=(aA·aP·aR(CN)₆)/aAPR(CN)₆ where a_(i) is the chemicalactivity of the i^(th) species. As ΔG=0 and R and T are nonzeroconstants, Keq=1, and therefore, (aA·aP·aR(CN)₆)/aAPR(CN)₆=1 as well. Inmost conditions, the chemical activity of a species can be approximatedby the concentration of that species, so cA·cP·cR(CN)₆/cAPR(CN)₆=1,where c_(i) is the concentration of the i^(th) species.

As cA·cP·cR(CN)₆/cAPR(CN)₆=1, the introduction of an additional quantityof one or more species A⁺, P^(m+), or R(CN)₆ ^(n−) to the system mustresult in the precipitation of APR(CN)₆ from dissolved A⁺, P^(m+), orR(CN)₆ ^(n−) until the equilibrium constant Keq=cA·cP·cR(CN)₆/cAPR(CN)₆is again equal to one. For example, the dissolution of the CuHCF cathodeis described by the following expression: KCuFe(CN)₆=K⁺+Cu²⁺+Fe(CN)₆ ³⁻,and the corresponding equilibrium constant isKeq=(cK·cCu·cFe(CN)₆)/cCuHCF=1. Therefore, CuHCF will be less soluble ina concentrated K⁺ electrolyte than in pure water, as a higher cK mustresult in lower equilibrium cCu and cFe(CN)₆. FIG. 38 illustratesconfirmation of this result by measurement of the dissolved ferricyanideconcentration (cFe(CN)₆) in either pure water or 1 M KNO₃ (pH=2) byultraviolet-visible (UV-vis) absorption spectroscopy.

Following the same principle, the addition of either P^(m+) or R(CN)₆^(n−) to the electrolyte will also shift the chemical equilibrium tofavor less dissolution of the solid APR(CN)₆ phase. In the case of theCuHCF cathode with a Cu²⁺ electrolyte additive, this result has beenconfirmed by both UV-vis spectroscopy and by electrochemical testing ofelectrodes as illustrated in FIG. 39 and FIG. 40.

The same principles are valid for the case of the MnHCMn anode. Thismaterial hydrolyzes rapidly in pure water or dilute alkali saltsolutions. However, it is much more stable, and therefore capable ofreversible electrochemical cycling, in concentrated alkali saltsolutions such as saturated sodium perchlorate. Furthermore, enhancedstability is observed upon the addition of Mn²⁺ to the electrolyte. Asimilar effect can also be achieved by the addition of CN⁻ anions to theelectrolyte, as their presence shifts the equilibrium towards MnHCMn,and away from a hydrolyzed product and dissolved CN⁻ anions. Theseresults for the stabilization of the MnHCMn anode, in combination withthose for the CuHCF cathode demonstrate that the general concept of aP²⁺ electrolyte additive to enhance the stability of a TMCCC applies toboth anodes and cathodes.

Electrode Life Extension Method 2: Combination of P²⁺ ElectrolyteAdditive with P Metal Anode

In most previous studies of TMCCC battery electrodes, another TMCCC or acapacitive carbon counter electrode was chosen. In a 1983 patent, Itayaet al briefly describe the use of a TMCCC cathode in combination with ametallic zinc anode in an aqueous NH₄Cl electrolyte. Metallic anodesoperate by dissolution during oxidation (discharge) and byelectroplating of the metal from cations in solution during reduction(charge).

The choice of a metallic anode P for use in an electrolyte containing aP^(m+) additive and a TMCCC cathode of the general formula APR(CN)₆ isadvantageous for at least two reasons. First, the presence of P^(m+) inthe electrolyte stabilizes the TMCCC cathode against dissolution.Second, the initial presence of P^(m+) in the electrolyte allows thebattery to start in a discharged state. Without the addition of P^(m+)to the electrolyte, no electrodeposition can occur at the anode.

FIG. 41 illustrates GCPL of the CuHCF cathode against a metallic Cuanode in an electrolyte containing 100 mM Cu(NO₃)₂. Analogous systemsinclude, but are not limited to nickel hexacyanoferrate/Ni²⁺/Ni and zinchexacyanoferrate/Zn²⁺/Zn. Furthermore, the P^(m+)/P anode system neednot match the transition metal cation found in the TMCCC cathode. Forexample, the CuHCF cathode could be operated in an electrolytecontaining Zn²⁺ and a Zn metal anode.

Electrode Life Extension Method 3: Electroless Deposition of TMCCCCoatings

A method for the stabilization of TMCCC electrodes against dissolutionis the use of a conformal coating that limits their contact with water.However, for an electrode with a coating layer to be useful, the coatingmust be conductive to alkali cations such as Na⁺ and K⁺, or it willprevent the charge and discharge of the TMCCC. Few materials systems arecapable of rapid Na⁺ or K⁺ conduction at room temperature.

Regular Prussian Blue is much less soluble than many of its analogues.Also, reduced Prussian Blue analogues/TMCCCs have been observed to beless soluble than oxidized ones. In addition, electrochemical oxidationof mixed-valent KFeIIIFeII(CN)₆ to Berlin Green (FeIIIFeIII(CN)₆) occursat a higher potential than the analogous oxidation of TMCCC cathodessuch as CuHCF. Therefore, if a CuHCF electrode is coated with a thin,conformal film of reduced, insoluble Prussian Blue, the CuHCF electrodemay undergo electrochemical cycling as usual. If the Prussian Bluecoating is continuous and conformal, the CuHCF electrode will notdissolve; however, the high ionic conductivity of Prussian Blue allowsthe electrode to still operate at high rates.

Because the oxidation potential of Prussian Blue to Berlin Green ishigher than the oxidation potential of CuHCF, a film of Prussian Bluecan be easily deposited onto CuHCF by an electroless reductiveprecipitation method.

The reduction of Berlin Green to Prussian Blue is analogous to thereduction of the CuHCF cathode, as in each case, carbon-coordinated ironin the framework crystal structure is reduced from Fe³⁺ to Fe²⁺. UnlikePrussian Blue, fully oxidized Berlin Green is sparingly soluble, sodilute solutions of Fe³⁺ and Fe(CN)₆ ³⁻ can be readily prepared.Electroless deposition of Prussian Blue onto a low-potential electrode(such as CuHCF) will occur from a dilute solution containing Fe³⁺ andFe(CN)₆ ³⁻ if that electrode has a potential below that of the reductionof Fe³⁺ to Fe²⁺ (V0=0.771 V vs. SHE). In the case of a CuHCF electrode,this occurs by the following two-step mechanism:

K₂CuFeII(CN)₆ +x·Fe³⁺=K_(2-x)Cu[FeII(CN)₆]_(1-x)[FeIII(CN)₆]_(x)+x·(Fe²⁺+K⁺)

x·(K⁺+Fe²⁺+Fe(CN)₆ ³⁻=KFeIIIFeII(CN)₆

It is reasonable to expect that the reduction of Fe³⁺ results in theformation of a thin film of Prussian Blue on the surface of the CuHCFelectrode, as there is widespread precedent for the electrodeposition ofPrussian Blue films by the same mechanism: reduction of iron cations,and subsequent Prussian Blue precipitation, from a dilute aqueoussolution of Fe³⁺ and Fe(CN)₆ ³⁺.

The oxidation of Prussian Blue to Berlin Green occurs at a higherpotential than the reaction potential of CuHCF (FIG. 42). This meansthat CuHCF electrodes in electrical contact with Prussian Blue can becycled without oxidizing the Prussian Blue to the more soluble BerlinGreen. Therefore, a conformal coating of insoluble Prussian Blueprevents the slow dissolution of CuHCF electrodes.

This technique offers several advantages: electroless deposition ofPrussian Blue is fast and inexpensive; alkali ion transport in PrussianBlue is extremely rapid; and the same technique could be used tostabilize TMCCC anodes (in fact, it could be used on any electrodefamily).

Methods:

-   -   1 cm² slurry electrodes containing 5 mg of CuHCF were prepared        using the standard methods described herein.    -   An aqueous deposition solution of 2 mM Fe(NO₃)₃ and 2 mM        K₃Fe(CN)₆ was prepared.    -   As-synthesized CuHCF has an open circuit potential near 1.05 V        vs. SHE, too high for electroless deposition of Prussian Blue        from Fe³⁺ and Fe(CN)₆ ³⁻. Thus, a preparative electrochemical        reduction to 0.7 V was performed by galvanostatic discharge at a        1C rate in 1 M KNO₃ (pH=2).    -   The discharged CuHCF electrodes were washed, dried, and then        placed in 10 mL of the deposition solution for 30 minutes. They        were then washed, dried, and inserted into batteries for        testing.    -   Though the fresh electrodes were black (due to the carbon in the        slurry), after exposure to the deposition, the electrodes        appeared slightly bluish.

The CuHCF electrodes were cycled at 1C between 0.8 and 1.05 V against aAg/AgCl reference electrode and an activated charcoal counter electrodein 15 mL of 1 M KNO₃ (pH=2).

Before exposure to the deposition solution, CuHCF electrodes weredischarged to 0.7 V vs. SHE. After deposition of the Prussian Bluecoating for 30 minutes, their open circuit potential was found to be0.85 V vs. SHE. From the previously reported galvanostatic potentialprofile of CuHCF (FIG. 33) this corresponds to a charge fraction ofabout 5% for the CuHCF, or 3 mAh/g based on its specific capacity of 60mAh/g. As each sample contained 5 mg CuHCF, the total charge expendedduring Prussian Blue deposition was 15 μAh. From the 10.16 Å latticeparameter of Prussian Blue and the planar geometric area of theelectrode, this total charge corresponds to the deposition of a filmwith a thickness of 1.1 μm. However, as the electrode is extremely roughwith a larger true surface area than its planar one, a true PrussianBlue coating thickness of ˜500 nm is reasonable.

The deposition of a Prussian Blue coating consistently improved thecapacity retention of the CuHCF electrode. The fractional capacityretention of two control samples and four samples with Prussian Bluecoatings is shown in FIG. 43. The improvement of CuHCF capacityretention is reproducibly achieved using the Prussian Blue coating step.Improving the completeness of the coverage of the conformal PrussianBlue coating through optimization of the coating procedure will furtherimprove the magnitude and reliability of the stabilizing effect of thecoating layer.

In some publications in which thin films of Prussian Blue areelectrodeposited, a supporting electrolyte such as 0.1 M KCl or K₂SO₄ isused. This aids electrodeposition, as the ionic conductivity of thesolution is much higher in the presence of a more concentrated salt. Todetermine whether or not a supporting electrolyte enhances theelectroless deposition of Prussian Blue coatings on CuHCF electrodes,the coating step was performed in the same 2 mM Fe³⁺/Fe(CN)₆ ³⁻solution, with 0.1 M KNO₃ added. At the end of the coating step, theelectrodes were washed and dried. Their color remained black, and didnot show evidence of a bluish tint. As shown in FIG. 34, there is noimprovement in the capacity retention.

FIG. 44 illustrates that the presence of excess K⁺ in the depositionsolution prevents the rapid growth of the Prussian Blue film on CuHCF.This result can be qualitatively explained be the presence of K⁺ on theright side of the first step of the deposition mechanism. The presenceof excess K⁺ shifts the equilibrium to the left side, so the CuHCF wouldhave to be reduced to a lower open circuit potential to reduce Fe³⁺ toFe²⁺ in the presence of excess K⁺.

Finally, the effect of the Prussian Blue coating step on the potentialprofile of the CuHCF electrode was examined. As shown in FIG. 45, thereis no discernible difference between the shapes of potential profiles ofsamples treated with the deposition solution and fresh control samples.

The morphologies of bare and Prussian Blue-coated CuHCF electrodes wereexamined using SEM (FIG. 46). The fresh sample is composed of easilydistinguished individual nanoparticles. However, the coated sample iscomposed of nanoparticles that are bound together in a continuouscoating layer. Exposure of the CuHCF electrode to the depositionsolution results in the formation of a conformal thin film of PrussianBlue. This film is directly responsible for improved electrode lifetimeduring battery operation because it acts as a barrier to CuHCFdissolution.

Electrode Life Extension Method 4: Combination of TMCCC Coating with P²⁺Electrolyte Additives

The method for electrode stabilization described in electrolessdeposition of TMCCC Coatings is now generalized. Other analogues besidesPrussian Blue itself may be used as a protective coating againstdissolution for another TMCCC, and a protective coating of the formulaAPR(CN)₆ may be used in combination with a P^(m+) electrolyte additivesand a P metal anode.

When the reduction potential of the APR(CN)₆ coating is higher than theoxidation potential of the TMCCC to be protected, then the sameelectroless deposition procedure as in the case of a Prussian Bluecoating can be used. For example, nickel hexacyanoferrate (NiHCF) has alower reaction potential than zinc hexacyanoferrate (ZnHCF). Electrolessdeposition of a conformal film of ZnHCF onto a NiHCF electrode willoccur spontaneously if that electrode is placed in a solution containingZn²⁺ and Fe(CN)₆ ³⁻:

K₂NiFeII(CN)₆+Zn²⁺+FeIII(CN)₆ ³⁻→KNiFeIII(CN)₆+KZnFeII(CN)₆

Or, in the case of materials not containing excess potassium in the Asites in their structures:

K₂Ni₃[FeII(CN)₆]²+3Zn²⁺+2FeIII(CN)₆³⁻→Ni₃[FeIII(CN)₆]²+K₂Zn³[FeII(CN)₆]²

This reaction occurs spontaneously because ZnHCF and other TMCCCs areless soluble when reduced than when oxidized. Therefore, for thisreaction to yield a conformal thin film on the electrode, but not theadditional spontaneous precipitation of oxidized ZnHCF particles, theconcentrations of the Zn²⁺ and FeIII(CN)₆ ³⁻ precursors must be greaterthan the saturation limit of reduced ZnHCF, but lower than thesaturation limit of oxidized ZnHCF. Or, more generally, the spontaneousprecipitation of a A1+xPR(CN)₆ film with a high reduction potential ontoa TMCCC electrode with a lower reduction potential will occur if theprecursors P^(m+) and R(CN)₆ ^(n−) are present in concentrations greaterthan the saturation limit of reduced A1+xPR(CN)₆, but lower than thesaturation limit of oxidized A_(x)PR(CN)₆.

The use of a Prussian Blue coating can be used in combination with a Feelectrolyte additive, but it is by the reduction potential of aqueousFe³⁺ to Fe²⁺ at 0.771 V. To avoid reversibly oxidizing and reducing theFe in the electrolyte, the potentials of the cathode and anode must bothremain below 0.771 V. For TMCCC cathodes with reaction potentials higherthan this, a different coating must be chosen. For example, a coating ofZnHCF can be combined with a Zn²⁺ electrolyte additive because Zn²⁺cannot be oxidized further in aqueous solutions. Furthermore, as theZn²⁺ electrolyte additive can be paired with a metallic Zn anode, ageneral cell of the following form can also be constructed: A TMCCCcathode of the general formula APR(CN)₆, coated by another TMCCC of thegeneral chemical formula AP′R′(CN)₆, with an electrolyte additiveP′^(m+) and a metallic anode P′. Combinations include a ZnHCFcoating/Zn²⁺ electrolyte additive/Zn metal anode and a NiHCFcoating/Ni²⁺ electrolyte additive/Ni metal anode. Furthermore, a ZnHCFcathode or ZnHCF-coated cathode could be paired with a Zn²⁺ electrolyteadditive and a galvanized steel anode, as the zinc in the galvanizedsurface layer would provide an adequate charge capacity.

Electrode Life Extension Method 5: Coating of Individual TMCCC Particleswith a TMCCC Shell

A protective coating of insoluble Prussian Blue or a TMCCC can beapplied not only to entire electrodes, but to the individual particlesthat compose the electrode. In one prior case, unrelated to the use ofTMCCCs as battery electrodes, nanoparticles of a TMCCC were coated witha conformal shell of another TMCCC. The advantage of this method ofelectrode stabilization is that if performed correctly, every particleof electrochemically active material has a conformal shell that preventsits dissolution. However, a larger total mass of protective layer isneeded because of the larger surface area.

Copper hexacyanoferrate was synthesized as described herein. Sodiumthiosulfate (Na₂S₂O₃), a reducing agent, was added by dropwise to thesolution containing the CuHCF nanoparticles 15 minutes after theirinitial precipitation. The Na₂S₂O₃ was added in a 0.8:1 molar ratio tothe potassium hexacyanoferrate precursor used to make the CuHCF. Duringthis process, the color of the solution changed from brown to purple.The low oxidation potential of Na₂S₂O₃ results in the reduction of theCuHCF nanoparticles. After this chemical reduction step, theelectrochemical potential of the CuHCF was below 0.771 V, low enough tospontaneously reduce Fe³⁺ to Fe²⁺.

The chemically reduced CuHCF was centrifuged and washed with water toremove excess Cu²⁺ left over from its precipitation. It was thenredispersed in pure water by sonication. Finally, by dropwise addition,a Prussian Blue deposition solution of 10 mM Fe(NO₃)₃ and 10 mMK₃Fe(CN)₆ was added to the solution of reduced CuHCF particles. Thissolution was slowly added until the molar ratio of the Fe(NO₃)₃ andK₃Fe(CN)₆ to the hexacyanoferrate in the CuHCF reached 1:4. The solutionchanged color from purple to dark blue, indicating that the Fe³⁺ wasreduced to Fe²⁺, and that the Fe²⁺ then reacted with the Fe(CN)₆ ³⁻ toform Prussian Blue. This process is analogous to the reduction of ironthat occurs during the exposure of electrodes containing CuHCF to thePrussian Blue deposition solution described herein. The rest of theelectrode preparation method was the same as described above.

The reduction by thiosulfate is necessary only because the CuHCF wassynthesized in a fully oxidized state, and its potential was too high toreduce the Fe³⁺ to Fe²⁺. In the case that some other TMCCC is chosen tobe the coating layer (for example, to be paired with a P²⁺ electrolyteadditive and a P metal anode), then the chemical reduction step of theelectrode nanoparticles may not be necessary, as described in thediscussion of a combination of TMCCC coating with P²⁺ electrolyteadditives for the case of a NiHCF electrode and a ZnHCF coating.

As shown in FIG. 47, an electrode containing CuHCF particles coated withPrussian Blue lost less than 1% of its capacity after 50 galvanostaticcycles at a 1C rate. In comparison, an electrode containing uncoatedCuHCF particles lost about 7% of its capacity after the same duration ofcycling. This result conclusively demonstrates that the coating ofindividual particles with Prussian Blue results protects them fromdissolution in the battery electrolyte.

The performance of the CuHCF electrode is similar with and without aconformal Prussian Blue coating of the individual particles. As shown inFIG. 48, the potential profiles of electrodes containing bare and coatedparticles are similar (FIG. 48a ).

Prussian Blue can be electrochemically reduced near 0.4 V vs. SHE. ThePrussian Blue coating is electrochemically active at low potential (FIG.17b ), confirming that the coating treatment indeed resulted in thesuccessful deposition of Prussian Blue coating. The ratio of theobserved capacities of CuHCF to Prussian Blue is about 4:1 between 0.2and 1.05 V vs. SHE. This is consistent with the 4:1 molar ratio of CuHCFto Prussian Blue precursors present during the coating procedure.

Electrode Life Extension Method 6: Coating of TMCCC Particles withPolymer Coatings by Redox Deposition

Other coatings besides TMCCCs may be used to protect a TMCCC batteryelectrode (or its constituent particles) from dissolution. Such acoating material must be conductive to cations such as Na⁺ or K⁺ so thatthe electrode can be charged and discharged, and it should not besignificantly soluble in aqueous electrolytes. If it has non-negligiblesolubility in aqueous electrolytes, then its dissolution products mustbe electrochemically inactive in the potential window of the anode andcathode of the battery.

A variety of mixed conducting polymers including polypyrroles andpolythiophenes are known to intercalate cations such as Nat They areinsoluble in aqueous electrolytes. Therefore, a conformal polymercoating can protect a TMCCC electrode from dissolution.

For example, CuHCF was synthesized by the standard method describedherein. Pyrrole was then added by slow, dropwise addition to thesolution in a 1:2 mass ratio with respect to the CuHCF already present.The solution immediately turned black, as the pyrrole was oxidized topolypyrrole upon contact with the CuHCF nanoparticles. The rest of theelectrode preparation method was the same as the standard method.

The use of a polypyrrole coating stabilizes the CuHCF againstdissolution in the battery electrolyte. As shown in FIG. 49, thecapacity of an electrode containing polypyrrole-coated CuHCFnanoparticles is completely stable for 50 galvanostatic cycles at a 1Crate in 1 M KNO₃ (pH=2). In comparison, a control electrode containinguntreated CuHCF loses about 7% of its capacity during cycling under thesame conditions.

The initial charge of polypyrrole-coated CuHCF shows a large,irreversible capacity. However, the electrode is completely stable incharge and discharge after the first few cycles. Little difference isobserved between the first discharge and the charge and discharge duringthe 20th cycle as illustrated in FIG. 50.

In some embodiments having a specific chemical formula for the PBAmaterial, e.g., copper hexacyanoferrate or manganese hexacyanomangate,depositing a coating may include polymerization of polythiophene.

Other Stabilization Methods

Below are described several additional methods for the stabilization ofTMCCCs against dissolution, and therefore, the extension of theoperational life of TMCCC electrodes.

Complexation with amines: a variety of amines have been shown to formstrong complexes with hexacyanoferrate, and therefore, can coordinatestrongly to the surface of a TMCCC particle to form a protective layer.These include simple diamines such as ethylene diamine, and largeraromatic amines such as Nile Blue. Furthermore, oxides of cyclic aminessuch as pyridine-n-oxide can be used to coat TMCCC particles. Additionof one or more of these amines during the synthesis of the TMCCC, or asan electrolyte additive, will result in a conformal surface coatinglayer that stabilizes the TMCCC against dissolution.

Anions of insoluble P2⁺ salts: soluble or trace-soluble alkali cationsalts such as sodium fluoride, carbonate, or oxalate can be added duringthe synthesis of a TMCCC, or as an electrolyte additive. During thedissolution of a TMCCC, the Pm⁺ cation hydrates and leaves the surfaceof the particle to enter the aqueous solution. Transition metal salts ofanions such as fluoride, carbonate, oxalate, and others are insoluble,and therefore, will react with the Pm+ at the surface of the TMCCCparticle to form an insoluble coating layer.

Thin films of insoluble transition metal sulfides such as CdS, Cu₂S,MnS, and ZnS are commonly fabricated for semiconductor devices includingphotodiodes. Deposition of these films from aqueous solution can beeasily accomplished by reaction of a transition metal cation Pm⁺ with asulfide precursor such as thiourea, thiosulfate, or sodium sulfide. Manytransition metal sulfides are good sodium ion conductors, so a metalsulfide coating of a TMCCC will protect it against dissolution whilestill allowing it to react electrochemically. These metal sulfides areunstable against hydrolysis at high potentials, and are most fit for useon TMCCC anodes with reaction potentials near or below SHE.

Similarly, small molecules containing thiol groups can coordinate to thetransition metal cations Pm+ on the surface of a TMCCC particle.Examples include simple thiols such as decanethiol, and more complicatedmolecules such as cysteine.

Additionally, extremely thin (5-10 nm) conformal layers of metal oxidessuch as Al₂O₃, SiO₂, and TiO₂ can be readily grown on the surfaces ofTMCCC nanoparticles using a sol-gel decomposition process fromorganometallic precursors. For example, in the case of SiO₂, the slowaddition of dilute tetraethyl orthosilicate (silicon tetraethoxide) tothe aqueous solution containing newly synthesized TMCCC particles willresult in the hydrolysis as polymerization of SiO₂ nanoparticles, whichform a thin, continuous film on the surface of the TMCCC particle. Theseoxides are completely insoluble in water, so they provide a robustbarrier to dissolution of the TMCCC. In addition, as they are extremelythin, and in some cases (such as Al₂O₃) have good Na+ conductivity, theydo not strongly limit the transport of alkali cations in and out of theTMCCC during electrochemical cycling.

In the discussion regarding additives and coatings, there aredescriptions of situations in which an additive to the electrolyte“sticks” bonds, or otherwise attaches to a surface of an electrodematerial and thereby form a coating. Additives are described assituations in which soluble chemical components of the electrode areadded to the electrolyte which does not result in a coating but doesreduce/prevent dissolution. There are also complex situations in which aPrussian Blue coating is applied to an electrode and then components ofthe coating are added to the electrolyte to stabilize and resistdissolution which slows/eliminates a rate of capacity loss. A coating ofan electrode may occur after an electrode is completely formed, orconstituent materials that will be used to form the electrode are coatedbefore the electrode is formed. The material(s) added to the electrolytethat is/are used to form a coating (distinguished from additives as aclass of substances added to the electrolyte that directly stabilize theelectrode) are not referred to herein as additives. Thesecoating-forming material(s) bond to the surface of the electrode to formthe stabilizing coating.

In some situations, for example Pyridine-N-oxide and the thiols andorganic molecules that stick to a surface of an electrode. Thesematerials are classified herein as coatings and not polymers as they donot bond together (polymerize) into big polymer strands. A hybridsolution includes attachment of small molecules onto a surface of theelectrode, or electrode constituents, and then polymerizing these smallmolecules all together into a single polymer coating for the electrode.

When coating a PBA electrode with a conformal coating layer of a PBAcoating material, the disclosed embodiments preferably use a differentPBA for the coating than is used for the electrode. The coating materialis selected to be more stable than the electrode material, and/or thecoating material allows a use of A, P, or R(CN)₆ electrolyte additivesin way that is better (e.g., less expensive, more stable, or the like).In these conformal coatings, the “P” transition metal cations of theelectrode and of the coating may be the same or different cations, andwhen the same the coating PBA has a different A and/or R(CN)₆ material.

In the case of additives, some embodiments provide for electrodes havingmultiple P transition metal cations (e.g., P1 and P2). The electrolytemay be presaturated with a P1 additive, a P2 additive, or both a P1additive and a P2 additive. For more than 2 P transition metal cationsin the electrode, all the different permutations of one or morecorresponding additive may be employed. In some cases the electrolyteadditive may be of a P transition metal cation that is not present in anelectrode of the system.

The system and methods above have been described in general terms as anaid to understanding details of preferred embodiments of the presentinvention. In the description herein, numerous specific details areprovided, such as examples of components and/or methods, to provide athorough understanding of embodiments of the present invention. Somefeatures and benefits of the present invention are realized in suchmodes and are not required in every case. One skilled in the relevantart will recognize, however, that an embodiment of the invention can bepracticed without one or more of the specific details, or with otherapparatus, systems, assemblies, methods, components, materials, parts,and/or the like. In other instances, well-known structures, materials,or operations are not specifically shown or described in detail to avoidobscuring aspects of embodiments of the present invention.

Reference throughout this specification to “one embodiment”, “anembodiment”, or “a specific embodiment” means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment of the present invention and notnecessarily in all embodiments. Thus, respective appearances of thephrases “in one embodiment”, “in an embodiment”, or “in a specificembodiment” in various places throughout this specification are notnecessarily referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics of any specificembodiment of the present invention may be combined in any suitablemanner with one or more other embodiments. It is to be understood thatother variations and modifications of the embodiments of the presentinvention described and illustrated herein are possible in light of theteachings herein and are to be considered as part of the spirit andscope of the present invention.

It will also be appreciated that one or more of the elements depicted inthe drawings/figures can also be implemented in a more separated orintegrated manner, or even removed or rendered as inoperable in certaincases, as is useful in accordance with a particular application.

Additionally, any signal arrows in the drawings/Figures should beconsidered only as exemplary, and not limiting, unless otherwisespecifically noted. Furthermore, the term “or” as used herein isgenerally intended to mean “and/or” unless otherwise indicated.Combinations of components or steps will also be considered as beingnoted, where terminology is foreseen as rendering the ability toseparate or combine is unclear.

The foregoing description of illustrated embodiments of the presentinvention, including what is described in the Abstract, is not intendedto be exhaustive or to limit the invention to the precise formsdisclosed herein. While specific embodiments of, and examples for, theinvention are described herein for illustrative purposes only, variousequivalent modifications are possible within the spirit and scope of thepresent invention, as those skilled in the relevant art will recognizeand appreciate. As indicated, these modifications may be made to thepresent invention in light of the foregoing description of illustratedembodiments of the present invention and are to be included within thespirit and scope of the present invention.

Thus, while the present invention has been described herein withreference to particular embodiments thereof, a latitude of modification,various changes and substitutions are intended in the foregoingdisclosures, and it will be appreciated that in some instances somefeatures of embodiments of the invention will be employed without acorresponding use of other features without departing from the scope andspirit of the invention as set forth. Therefore, many modifications maybe made to adapt a particular situation or material to the essentialscope and spirit of the present invention. It is intended that theinvention not be limited to the particular terms used in followingclaims and/or to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include any and all embodiments and equivalents falling within thescope of the appended claims. Thus, the scope of the invention is to bedetermined solely by the appended claims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A rechargeable electrochemical device,comprising: a first electrode; a second electrode; an electrolytecoupled with said electrodes; and a first additive in communication withsaid electrolyte; wherein a first particular one electrode of saidelectrodes includes a first variable potential material; and whereinsaid first additive participates in a first predetermined side-reactionwith a first single one of said electrodes degrading a chargingefficiency of said first single one of said electrodes for a duration ofsaid first predetermined side-reaction.
 2. The rechargeableelectrochemical device of claim 1 wherein said duration is preconfiguredfor a reduction in a relative state-of-charge imbalance between saidelectrodes after charging of said electrodes.
 3. The rechargeableelectrochemical device of claim 1 wherein said first particular oneelectrode includes a first transition metal cyanide coordinationcompound (TMCCC).
 4. The rechargeable electrochemical device of claim 1further comprising a second additive in communication with saidelectrolyte wherein a second particular one electrode of saidelectrodes, different from said first particular one electrode of saidelectrodes, includes a second variable potential material; and whereinsaid second additive participates in a second predeterminedside-reaction with a second single one of said electrodes.
 5. Therechargeable electrochemical device of claim 4 wherein said secondparticular one electrode includes a second transition metal cyanidecoordination compound (TMCCC).
 6. The rechargeable electrochemicaldevice of claim 3 wherein said TMCCC material includes a compositionhaving the general chemical formulaA_(x)M_(y)[R(CN)_(6-j)L_(j)]_(z).nH₂O, wherein: A includes one or morecations; M includes one or more metal cations; R includes one or moretransition metal cations; and L is a ligand substituted in the place ofa CN⁻ ligand; where 0≤x≤2; 0<y≤4; 0<z≤1; 0≤j<6; and 0≤n≤5.
 7. Therechargeable electrochemical device of claim 3 wherein said electrolyteincludes a total electrolyte volume V including a first quantity ofwater comprising a first fraction V1 of said total electrolyte volume Vand including a second quantity of one or more organic cosolventstogether comprising a second fraction V2 of said total electrolytevolume V, wherein V1>0.02, and wherein said electrolyte consistsessentially of a single phase.
 8. The rechargeable electrochemicaldevice of claim 7 wherein V2>V1.
 9. The rechargeable electrochemicaldevice of claim 8 wherein said one or more organic solvents includes asolvent containing a cyanide group.
 10. The rechargeable electrochemicaldevice of claim 9 wherein said second quantity V2 includes acetonitrile.11. The rechargeable electrochemical device of claim 8 wherein said oneor more organic cosolvents includes a solvent containing a sulfonegroup.
 12. The rechargeable electrochemical device of claim 11 whereinsaid sulfone group includes sulfolane.
 13. The rechargeableelectrochemical device of claim 8 wherein a concentration of saidadditive is included within a range of 10 to 10,000 parts per million.14. The rechargeable electrochemical device of claim 8 wherein saidadditive includes one or more organic molecules.
 15. The rechargeableelectrochemical device of claim 14 wherein said one or more organicmolecules are configured to participate in a reversible electrochemicalredox reaction at one or more of said electrodes during an applicationof charging energy to said electrodes.
 16. The rechargeableelectrochemical device of claim 15 wherein said additive includes aquinone group.
 17. The rechargeable electrochemical device of claim 14wherein said one or more organic molecules are configured to participatein an irreversible electrochemical redox reaction at one or more of saidelectrodes during an application of charging energy to said electrodes.18. The rechargeable electrochemical device of claim 17 wherein saidirreversible electrochemical redox reaction results in a polymerizationof said one or more organic molecules.
 19. The rechargeableelectrochemical device of claim 18 wherein said one or more organicmolecules include a pyrrole group.
 20. The rechargeable electrochemicaldevice of claim 8 wherein said additive includes a transition metalsalt.
 21. The rechargeable electrochemical device of claim 20 whereinsaid salt is configured to participate in a reversible electrochemicalredox reaction at one or more of said electrodes during an applicationof charging energy to said electrodes.
 22. The rechargeableelectrochemical device of claim 20 wherein said salt is configured toparticipate in an irreversible reaction at one or more of saidelectrodes during an application of charging energy to said electrodes.23. The rechargeable electrochemical device of claim 22 wherein saidsalt includes a transition metal cation.
 24. The rechargeableelectrochemical device of claim 22 wherein said salt includes atransition metal polyanion.
 25. The rechargeable electrochemical deviceof claim 8 wherein said additive includes an organometallic molecule.26. The rechargeable electrochemical device of claim 25 wherein saidorganometallic molecule is configured to participate in a reversibleelectrochemical redox reaction at one or more of said electrodes. 27.The rechargeable electrochemical device of claim 25 wherein saidorganometallic molecule includes a metallocene.
 28. The rechargeableelectrochemical device of claim 25 wherein said organometallic moleculeis configured to participate in an irreversible reaction at one or moreof said electrodes during an application of charging energy at saidelectrodes.
 29. The rechargeable electrochemical device of claim 8wherein said additive includes a surfactant.
 30. A method for reducing arelative state-of-charge imbalance of a set of electrodes of arechargeable electrochemical device during a recharging process, the setof electrodes coupled to an electrolyte and wherein at least oneelectrode of the set of electrodes includes a first variable potentialmaterial, comprising: a) performing the recharging process for arecharging duration which charges the electrodes at different relativerates to tend to produce a relative state-of-charge imbalance for theset of electrodes; and b) reducing said relative state-of-chargeimbalance by interfering with a charging of at least one electrode ofthe set of electrodes.